Re: [RFC PATCH 1/9] apple-gmux: use cpu_to_be32 instead of manual reorder

[Orlando Chamberlain]

On Fri, 10 Feb 2023 20:19:27 +0100
Hans de Goede <hdegoede@xxxxxxxxxx> wrote:

> Hi,
> 
> On 2/10/23 20:09, Hans de Goede wrote:
> > Hi,
> > 
> > On 2/10/23 05:48, Orlando Chamberlain wrote:  
> >> Currently it manually flips the byte order, but we can instead use
> >> cpu_to_be32(val) for this.
> >>
> >> Signed-off-by: Orlando Chamberlain <orlandoch.dev@xxxxxxxxx>
> >> ---
> >>  drivers/platform/x86/apple-gmux.c | 18 ++----------------
> >>  1 file changed, 2 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/drivers/platform/x86/apple-gmux.c
> >> b/drivers/platform/x86/apple-gmux.c index
> >> 9333f82cfa8a..e8cb084cb81f 100644 ---
> >> a/drivers/platform/x86/apple-gmux.c +++
> >> b/drivers/platform/x86/apple-gmux.c @@ -94,13 +94,7 @@ static u32
> >> gmux_pio_read32(struct apple_gmux_data *gmux_data, int port)
> >> static void gmux_pio_write32(struct apple_gmux_data *gmux_data,
> >> int port, u32 val) {
> >> -	int i;
> >> -	u8 tmpval;
> >> -
> >> -	for (i = 0; i < 4; i++) {
> >> -		tmpval = (val >> (i * 8)) & 0xff;
> >> -		outb(tmpval, gmux_data->iostart + port + i);
> >> -	}
> >> +	outl(cpu_to_be32(val), gmux_data->iostart + port);
> >>  }
> >>  
> >>  static int gmux_index_wait_ready(struct apple_gmux_data
> >> *gmux_data)  
> > 
> > The ioport / indexed-ioport accessed apple_gmux-es likely are (part
> > of?) LPC bus devices . Looking at the bus level you are now
> > changing 4 io accesses with a size of 1 byte, to 1 32 bit io-access.
> > 
> > Depending on the decoding hw in the chip this may work fine,
> > or this may work not at all.
> > 
> > I realized that you have asked for more testing, but most surviving
> > macbooks from the older apple-gmux era appear to be models without
> > a discrete GPU (which are often the first thing to break) and thus
> > without a gmux.
> > 
> > Unless we get a bunch of testers to show up, which I doubt. I would
> > prefer slightly bigger / less pretty code and not change the
> > functional behavior of the driver on these older models.  
> 
> A quick follow up on this, I just noticed that only the pio_write32
> is doing the one byte at a time thing:
> 
> static u32 gmux_pio_read32(struct apple_gmux_data *gmux_data, int
> port) {
>         return inl(gmux_data->iostart + port);
> }
> 
> static void gmux_pio_write32(struct apple_gmux_data *gmux_data, int
> port, u32 val)
> {
>         int i;
>         u8 tmpval;
> 
>         for (i = 0; i < 4; i++) {
>                 tmpval = (val >> (i * 8)) & 0xff;
>                 outb(tmpval, gmux_data->iostart + port + i);
>         }
> }
> 
> And if you look closely gmux_pio_write32() is not swapping
> the order to be32 at all, it is just taking the bytes
> in little-endian memory order, starting with the first
> (index 0) byte which is the least significant byte of
> the value.
> 
> On x86 the original code is no different then doing:
> 
> static void gmux_pio_write32(struct apple_gmux_data *gmux_data, int
> port, u32 val)
> {
>         u8 *data = (u8 *)&val;
>         int i;
> 
>         for (i = 0; i < 4; i++)
>                 outb(data[i], gmux_data->iostart + port + i);
> }
> 
> So yeah this patch is definitely wrong, it actually swaps
> the byte order compared to the original code. Which becomes
> clear when you look the weird difference between the read32 and
> write32 functions after this patch.
> 
> Presumably there is a specific reason why gmux_pio_write32()
> is not already doing a single outl(..., val) and byte-ordering
> is not the reason.
> 
> Regards,
> 
> Hans

Sounds like it may be better to just drop this patch as there's very
little benefit for the risk of causing a regression.

> 
> 
> 
> >> @@ -177,16 +171,8 @@ static u32 gmux_index_read32(struct
> >> apple_gmux_data *gmux_data, int port) static void
> >> gmux_index_write32(struct apple_gmux_data *gmux_data, int port,
> >> u32 val) {
> >> -	int i;
> >> -	u8 tmpval;
> >> -
> >>  	mutex_lock(&gmux_data->index_lock);
> >> -
> >> -	for (i = 0; i < 4; i++) {
> >> -		tmpval = (val >> (i * 8)) & 0xff;
> >> -		outb(tmpval, gmux_data->iostart + GMUX_PORT_VALUE
> >> + i);
> >> -	}
> >> -
> >> +	outl(cpu_to_be32(val), gmux_data->iostart +
> >> GMUX_PORT_VALUE); gmux_index_wait_ready(gmux_data);
> >>  	outb(port & 0xff, gmux_data->iostart + GMUX_PORT_WRITE);
> >>  	gmux_index_wait_complete(gmux_data);  
> >   
>