Re: [PATCH] dmapool: push new blocks in ascending order

[Bryan O'Donoghue]

On 24/02/2023 18:24, Keith Busch wrote:
> On Thu, Feb 23, 2023 at 12:41:37PM -0800, Andrew Morton wrote:
> > On Tue, 21 Feb 2023 11:07:32 -0700 Keith Busch <kbusch@xxxxxxxxxx> wrote:
> >
> > > On Tue, Feb 21, 2023 at 10:02:34AM -0800, Christoph Hellwig wrote:
> > > > On Tue, Feb 21, 2023 at 08:54:00AM -0800, Keith Busch wrote:
> > > > > From: Keith Busch <kbusch@xxxxxxxxxx>
> > > > >
> > > > > Some users of the dmapool need their allocations to happen in ascending
> > > > > order. The recent optimizations pushed the blocks in reverse order, so
> > > > > restore the previous behavior by linking the next available block from
> > > > > low-to-high.
> > > >
> > > > Who are those users?
> > > >
> > > > Also should we document this behavior somewhere so that it isn't
> > > > accidentally changed again some time in the future?
> > >
> > > usb/chipidea/udc.c qh_pool called "ci_hw_qh".
> >
> > It would be helpful to know why these users need this side-effect.  Did
> > the drivers break?   Or just get slower?
>
> The affected driver was reported to be unusable without this behavior.
>   
> > Are those drivers misbehaving by assuming this behavior?   Should we
>
> I do think they're using the wrong API. You you shouldn't use the dmapool if
> your blocks need to be arranged in a contiguous address order. They should just
> directly use dma_alloc_coherent() instead.
>
> > require that they be altered instead of forever constraining the dmapool
> > implementation in this fashion?
>
> This change isn't really constraining dmapool where it matters. It's just an
> unexpected one-time initialization thing.
>
> As far as altering those drivers, I'll reach out to someone on that side for
> comment (I'm currently not familiar with the affected subsystem).

We can always change this driver, I'm fine to do that in-parallel/instead.

The symptom we have is a silent failure absent this change so, I just 
wonder are we really the _only_ code path that would be affected absent 
the change in this patch ?

---
bod