Re: [PATCH] swiotlb: set IO TLB segment size via cmdline

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.09.2021 15:37, Roman Skakun wrote:
>>> From: Roman Skakun <roman_skakun@xxxxxxxx>
>>>
>>> It is possible when default IO TLB size is not
>>> enough to fit a long buffers as described here [1].
>>>
>>> This patch makes a way to set this parameter
>>> using cmdline instead of recompiling a kernel.
>>>
>>> [1] https://www.xilinx.com/support/answers/72694.html
>>
>>  I'm not convinced the swiotlb use describe there falls under "intended
>>  use" - mapping a 1280x720 framebuffer in a single chunk?
> 
> I had the same issue while mapping DMA chuck ~4MB for gem fb when
> using xen vdispl.
> I got the next log:
> [ 142.030421] rcar-fcp fea2f000.fcp: swiotlb buffer is full (sz:
> 3686400 bytes), total 32768 (slots), used 32 (slots)
> 
> It happened when I tried to map bounce buffer, which has a large size.
> The default size if 128(IO_TLB_SEGSIZE) * 2048(IO_TLB_SHIFT) = 262144
> bytes, but we requested 3686400 bytes.
> When I change IO_TLB_SEGSIZE to 2048. (2048(IO_TLB_SEGSIZE)  *
> 2048(IO_TLB_SHIFT) = 4194304bytes).
> It makes possible to retrieve a bounce buffer for requested size.
> After changing this value, the problem is gone.

But the question remains: Why does the framebuffer need to be mapped
in a single giant chunk?

>>  In order to be sure to catch all uses like this one (including ones
>>  which make it upstream in parallel to yours), I think you will want
>>  to rename the original IO_TLB_SEGSIZE to e.g. IO_TLB_DEFAULT_SEGSIZE.
> 
> I don't understand your point. Can you clarify this?

There's a concrete present example: I have a patch pending adding
another use of IO_TLB_SEGSIZE. This use would need to be replaced
like you do here in several places. The need for the additional
replacement would be quite obvious (from a build failure) if you
renamed the manifest constant. Without renaming, it'll take
someone running into an issue on a live system, which I consider
far worse. This is because a simple re-basing of one of the
patches on top of the other will not point out the need for the
extra replacement, nor would a test build (with both patches in
place).

Jan