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

[Will Deacon <will@kernel.org>]

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.

Will