Re: [PATCH 2.6-git] SPI core refresh

[Mark Underwood]

--- Vitaly Wool <vwool@xxxxxxxxxxxxx> wrote:

> David Brownell wrote:
> 
> >On Wednesday 30 November 2005 11:17 pm, Vitaly Wool wrote:
> >  
> >
> >>Mark Underwood wrote:
> >>
> >>    
> >>
> >>>>However, there also are some advantages of our core compared to David's I'd like to mention
> >>>>
> >>>>- it can be compiled as a module
> >>>>        
> >>>>
> >>>So can David's. You can use BIOS tables in which case you must compile the SPI core into the
> >>>kernel but you can also use spi_new_device which allows the SPI core to be built as a module
> (and
> >>>is how I am using it).
> >>>      
> >>>
> >>You limit the functionality, so it's not the case.
> >>    
> >>
> >
> >As noted in my comparison of last week (you're still ignoring that):
> >
> > - Mine lets board-specific device tables be declared in the
> >   relevant arch_setup() thing (board-*.c).  Both frameworks allow
> >   later board specific code to dynamically declare the devices,
> >   with binary (Dave's) or parsed-text (Dmitry's) descriptions. 
> >
> >What Mark said was that in this case he used the "late" init.  You seem
> >to be saying he's not allowed to do that.  Which is nonsense; there are
> >distinct mechanisms for the good reason that "late" init doesn't work
> >so well without dynamic discovery ... which SPI itself doesn't support.
> >Hence the need for board-specific "this hardware exists" tables.
> >
> >  
> >
> Can you please clarify what you mean here? Better even if Mark describes 
> what he does. The ideal situation would be if he posted a patch.
> 

Here it is :)

--- /dev/null	2005-12-02 17:53:19.000000000 +0000
+++ parport_plat.c	2005-11-26 16:57:24.000000000 +0000
@@ -0,0 +1,66 @@
+/*
+ *  parport_plat.c - Platfrom file describing the SPI parport adapter
+ *                   board that has an eeprom on it.
+ *
+ *  Copyright (C) 2005 Mark Underwood
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+
+/* This is an example of how to declare and register SPI devices which
+ * are not declared at arch init time.
+ */
+
+#include <linux/device.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include "../../include/linux/spi/spi.h"
+
+struct spi_board_info parport_eeprom_board = {
+	.modalias	= "eeprom",
+	.max_speed_hz	= 500000,
+	.bus_num	= 1,
+	.chip_select	= 2,
+};
+
+struct spi_device *device;
+
+static int __init parport_plat_init(void)
+{
+	struct spi_master *master;
+
+	master = spi_busnum_to_master(parport_eeprom_board.bus_num);
+	if (master)
+	{
+		device = spi_new_device(master, &parport_eeprom_board);
+		if (device)
+			printk("Registerd to bus %s\n", device->dev.bus_id);
+		else
+		{
+			printk("Failed registering %s !\n", parport_eeprom_board.modalias);
+       			return -ENODEV;
+		}
+	}
+	else
+	{
+		printk("No master!\n");
+		return -ENODEV;
+	}
+
+	return 0;
+}
+
+static void __exit parport_plat_cleanup(void)
+{
+	spi_unregister_device(device);
+}
+
+module_init(parport_plat_init);
+module_exit(parport_plat_cleanup);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Mark Underwood basicmark(at)yahoo.com");
+MODULE_DESCRIPTION("SPI parport adapter with eeprom platform file");	


> >  
> >
> >>If there's more than one SPI controller onboard, spi_write_then_read 
> >>will serialize the transfers ...
> >>    
> >>
> >
> >Which, as has been pointed out, would be a trivial thing to fix
> >if anyone were actually to have a problem.  Sure it'd incur the
> >cost of a kmalloc on at least some paths -- serializing in the
> >slab layer instead! -- but that's one price of using convenience
> >helpers not performance oriented calls.
> >  
> >
> Well, most of the drivers will use that helpers I guess.
> The thing however is that if you try to implement this in a "clean" way 
> you will come to a sport of framework we've developed for memeory 
> allocations, as I've saild previously.
> 
> >
> >  
> >
> >>	 Moreover, if, say, two  
> >>kernel threads with different priorities are working with two SPI 
> >>controllers respectively *priority inversion* will happen.
> >>    
> >>
> >
> >That characteristic being inherited from semaphores (or were they
> >updated with RT_PREEMPT?), and being in common with most I/O queues
> >in the system.  Not something to blame on any line of code I wrote.
> >  
> >
> I think they weren't.
> The whole thing doesn't seem thought out nicely to me. The solution 
> exists, of course, and that is -- do somthing similar to what we did there.
> 

I know what priority inversion is and I can't see where I would happen in Davids SPI core. 
 
I'm sorry but your transfer technique is far more complex then it needs to be. Davids transfer
technique follows that of the USB subsystem, why do you think that SPI, which is _much simpler_,
requires this amount of complexity? 
 
In your subsystem for every adapter you _force_ and thread and a workqueue on that adapter :(,
most adapter drivers will not need either!

> >Oh, and I noticed a priority inversion in your API which shows
> >up with one SPI controller managing two devices.  Whoops!  I'd
> >far rather have such inversions be implementation artifacts; it's
> >easy to patch an implementation, hard to change all API users.
> >  
> >
> Not sure if I understood you. Can you please describe the situation when 
> this prio inversion happens?
> What priorities are you talking about? One controller is one thread, so 
> it's _one_ priority, consequently there's nothing to invert.
> As for your second statement, I don't argue. The fact however is that if 
> you implement the mehtod which corrects priority inverstion problems 
> your core will not be either so lightweight or so flexible. :)
> 

What priority inversion problem is there to fix in Davids core !?!

> >
> >  
> >
> >>>>- it's more adapted for use in real-time environments
> >>>>- it's not so lightweight, but it leaves less effort for the bus driver developer.
> >>>>        
> >>>>
> >>>But also less flexibility. A core layer shouldn't _force_ a policy
> >>>      
> >>>
> >>Nope, it's just a default policy.
> >>    
> >>
> >
> >One that every driver pays the price for.  Allocating a task even
> >when it doesn't need it; every call going through a midlayer that
> >wants to take over queue management policy; and more.  (Unless you
> >made a big un-remarked change in a patch you called "refresh"...)
> >  
> >
> It's not obvious that this price is high.
> Anyway, it's a point I should agree with; this functionality better be a 
> config option. Feel free to submit a patch, as you like to say.
> 
> >
> >  
> >
> >>>on a bus driver. I am currently developing an adapter driver for David's system and I
> wouldn't say
> >>>that the core is making me do things I think the core should do. Please could you provide
> examples
> >>>of where you think Davids SPI core requires 'effort'.
> >>>      
> >>>
> >>Main are
> >>- the need to call 'complete' in controller driver
> >>    
> >>
> >
> >So you think it's better to have consistent semantics be optional?
> >
> >That seems to be the notion behind your spi_transfer() call, which
> >can't decide whether it's going to be synchronous or asynchronous.
> >Instead, it decided to be error prone and be both.  :)
> >
> >
> >  
> >
> Not sure if I understood you here, sorry.
> 
> >>- the need to implement policy in controller driver
> >>    
> >>
> >
> >The "policy" in question is something that sometimes needs to
> >be board-specific -- priority to THAT device, synch with THIS
> >external signal, etc -- which is why I see it as a drawback
> >that you insist the core implement one policy.
> >  
> >
> Again, the policy can be overridden.

I thought in general a device driver should't force a policy on its use surely this goes double
(or more) for a driver core. 
 
Mark

> 
> Vitaly
> 



		
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