[PATCH 11/12] DMAENGINE: let PL08X memcpy TXDs wait

[Dan Williams]

On Thu, Sep 23, 2010 at 12:39 AM, Linus Walleij
<linus.ml.walleij at gmail.com> wrote:
> 2010/9/23 Dan Williams <dan.j.williams at intel.com>:
>> On Tue, Aug 31, 2010 at 5:12 AM, Linus Walleij
>> <linus.walleij at stericsson.com> wrote:
>>> This change makes the memcpy transfers wait for a physical channel
>>> to become available if no free channel is available when the job
>>> is submitted. When the first physical channel fires its tasklet,
>>> it will spin over the memcpy channels to see if one of these is
>>> waiting.
>>>
>>> This is necessary to get the memcpy semantics right: the generic
>>> memcpy API assume transfers never fail, and with our oversubscribed
>>> physical channels this becomes a problem: sometimes submit would
>>> fail. This fixes it by letting the memcpy channels pull a free
>>> channel ASAP.
>>>
>>> The slave channels shall however *fail* if no channel is available
>>> since the device will then either fall back to some PIO mode or
>>> retry.
>>>
>>
>> This patch does not sit right with me.  It seems a bit arbitrary that
>> memcpy operations will be queued while slave operations are failed.
>
> I was sort of suspecting a discussion around this...
>
> Background: the PL081 that I'm testing on only has *two* physical
> channels, they can be used for memcpy, slave transfers (RX/TX).
>
> My first option was to set one channel aside for memcpy and
> one for dev xfers and be done with it.
>
> But that would be devastating if
> I was only using them for memcpy or only devxfer, since it would
> slash performance in half. So I decided to let memcpy and
> dev xfers battle for the channels with oversubscription and exposing
> two virtual memcpy channels.
>
> Whereas the slave drivers I've written are prepared to handle the
> case where a transfer fails gracefully (and not treat it as an error)
> and fall back to IRQ/PIO mode, the memcpy tests in
> drivers/dma/dmatest.c does treat a failure to prep() or submit()
> as an error.

Yeah, that is a hold over from when the original ioatdma driver
performed memory allocations at submit time (fixed here a0587bcf).

>
> So for this reason I'm queueing memcpy requests until
> there is a channel ready.
>
> Are you suggesting I should rewrite the memcpy tests instead
> so they gracefully handle a failed prep() call, not treat it as an
> error and either:
>
> - retry or
> - copy with a regular klibc memcpy() call?

I was more thinking that at a minimum all slave drivers would follow
the same semantic and expect their submit requests to be queued or
fail, as it stands we have a mix of both across all slave
implementations.

>
>> Is there anyway to know at prep time whether a subsequent submit will
>> fail?  Are there any cases where a slave might want its operation
>> queued?
>>
>> The prep routine is meant to guarantee that all the resources for a
>> transaction have been acquired.  The only reason ->tx_submit() has a
>> return value is to support the net_dma usage model that uses opaque
>> cookies for tracking transactions.
>
> It is possible to do this at prep() time. However that assumes that every
> device transfer has code that executes this sequence:
>
> .device_prep_slave_sg()
> .tx_submit()
> .device_issue_pending()
>
> In direct succession. If there is an arbitrary delay between prep()
> and submit() (where I currently fail xfers) the physical channels
> may starve if prep() allocates them.

There is precedent, albeit a bit inelegant, for taking a lock in
prep() and dropping it in submit().  The ioatdma driver does this to
ensure in-order submission because the hardware caches the address of
the next descriptor in the ring.  Out-of-order submission would
require a hardware reset to clear that cache prep() and submit() are
not paired.

> If I can safely assume that prep() and .tx_submit() are in quick
> succession, I can reserve the physical channel at prep() time.
>

It is safe to actively enforce this as long as you never plan to
support the async_tx channel switching capability (only a few raid
drivers use this presently).  I'm converting to be an explicit opt-in
because most drivers don't need this and can use the cut down version
of dma_async_tx_descriptor.

> This seems to be the case in current code where only quick
> things like setting a callback etc is done between prep() and
> .tx_submit().
>
> So I'll spin the PL08x around to reserve channels on
> prep() instead.
>
> (By the way: we should rename .tx_submit() to just .submit()
> since in device context this can just as well be RX!)

The 'tx' in async_tx came "asynchronous transfer/transform"  where the
'x' is transfer or transform.  I'd also like to change drop the
'device_' from the method names and rename dma_async_tx_descriptor to
something shorter, but I'm not sure it is worth the thrash.

>> If we make tx_submit() fallable we should go back and ensure that all
>> usages are prepared to handle failure.
>
> OK I've implemented failure path for tx_submit() in all my
> drivers but I also have it for prep() of course. I hope I can keep
> that code... I'll take out the comments about allocation failure
> and move them to the prep() calls though.
>

Sounds good.

--
Dan