Re: [PATCH] iommu/arm-smmu-v3: Stop queue allocation retry at PAGE_SIZE

[Leo Jiang <leo.jiang1224@foxmail.com>]

On Tue, Apr 21, 2026 at 05:38:40PM +0100, Will Deacon wrote:

> On Tue, Apr 21, 2026 at 04:56:47PM +0100, Robin Murphy wrote:
> > On 18/04/2026 6:31 am, leo.jiang1224@foxmail.com wrote:
> > > From: LoserJL <leo.jiang1224@foxmail.com>
> > > 
> > > In arm_smmu_init_one_queue(), the loop reduces max_n_shift if
> > > dmam_alloc_coherent() fails. However, since dmam_alloc_coherent()
> > > allocates at least PAGE_SIZE, retrying with a smaller size after
> > > a PAGE_SIZE failure is logically redundant.
> > 
> > Says who? It's certainly not a guarantee offered by the DMA API itself, and
> > indeed some allocation paths can definitely still allocate less than a page
> > - e.g. anything which hits a per-device or global coherent pool.
> > 
> > > Moreover, if a sub-page retry were to succeed due to concurrent memory
> > > release, the hardware would be configured with a smaller queue depth
> > > despite a full page being allocated. This leads to inefficient memory
> > > usage and unnecessary hardware performance limitation.
> > > 
> > > Terminate the loop once qsz reaches PAGE_SIZE to ensure logical
> > > consistency and optimal hardware configuration.
> > 
> > That's really not an argument - even if an allocator does happen to
> > over-allocate for the requested size, that is hardly the caller's concern;
> > and as far as "optimal" queue sizes go in this case, those very much depend
> > on the number of CPUs issuing commands and volume of expected stall/PRI
> > events - in many cases PAGE_SIZE would already be far too small to really
> > work well.
> > 
> > Also note that if we _were_ to fail to allocate a PAGE_SIZE or smaller
> > queue, there would be very little chance of the subsequent allocation(s) for
> > the stream table succeeding, so realistically the driver is probably going
> > to end up failing to probe in such circumstances anyway.
> 
> That's all true, but tbf I think I just fscked up the comparison in
> d25f6ead162e ("iommu/arm-smmu-v3: Increase maximum size of queues") so
> I'm not against fixing that up even though the "rationale" given by
> Loser doesn't make a whole lot of sense.

Hi Will, Robin,

Thank you both for the detailed feedback.

Robin, you are absolutely correct. After a deeper look into the source code,
I see that allocations smaller than a page are indeed possible in certain
cases. My previous assumption about the DMA API's granularity was wrong,
and I appreciate the correction.

However, as Will noted that the current logic deviates from the original
intent, I have prepared a v2 to limit the queue allocation retry boundary
to PAGE_SIZE.

In v2, I have:
 - Updated my identity to Leo Jiang.
 - Removed the code comments as suggested.

I will send the v2 as a follow-up shortly.

Best regards,
Leo Jiang