[PATCH] ARM: mm: Fix ARCH_LOW_ADDRESS_LIMIT when CONFIG_ZONE_DMA

[Stefan Wahren]

Hi,

Am 03.06.23 um 15:58 schrieb Lukas Wunner:
> On Sat, Jun 03, 2023 at 02:59:53PM +0200, Arnd Bergmann wrote:
>> On Sat, Jun 3, 2023, at 14:38, Russell King (Oracle) wrote:
>>> On Sat, Jun 03, 2023 at 02:33:49PM +0200, Arnd Bergmann wrote:
>>>> Changing the low address limit by itself makes the swiotlb work
>>>> correctly, but for performance we also want to avoid having to
>>>> use the swiotlb at all when dealing with the zero page.
>>>
>>> How often is the zero page used for DMA _to_ a device (it should never
>>> be used for DMA _from_ a device.) ? It doesn't sound that useful to me,
>>> since it can only be used for writing zeros.
>>
>> It's possible that this doesn't happen all that much at all, the
>> bcm2835-dma driver is the only one that has a specific optimization
>> for this case, introduced in commit bf75703d0912d ("dmaengine:
>> bcm2835: Avoid accessing memory when copying zeroes").
>>
>> If the ZERO page can only be mapped through swiotlb, that
>> optimization is clearly counterproductive because it requires
>> making a copy of the zero page but doesn't actually skip
>> any transfers.
>>
>> I don't really understand what the spi driver is doing here,
>> but I can see that it still contains the code that originally
>> started the discussion about the change to the DMA driver,
>> so I assume it's still used that way.
>
> SPI is a bidirectional bus, i.e. you receive as many bytes as you write.
>
> When you're receiving an Ethernet frame from an SPI-attached network chip,
> you need to clock out bytes in order to clock in the Ethernet frame.
> The content of those bytes you clock out doesn't matter.  They can be
> zeroes or anything else.
>
> The Raspberry Pi's BCM2835 DMA controller is capable of synthesizing zeroes
> and writing them to the SPI controller's FIFO.  This reduces traffic on the
> memory bus because you don't need to copy data from memory to the FIFO.
>
> Unfortunately only a subset of the DMA channels has that capability.
> If the SPI controller happens to get only one of the inferior "lite"
> channels, it needs to fall back to copying data from memory.  To avoid
> having to map a page for that, it simply reuses the zero page.
>
> Hope that sheds some light on what the driver is doing there.

i'm bit a lost in this old discussion. Arnd sent a Reviewed-By to the patch.

Shall i send this to RMK's patch system?

Or is the patch not sufficient enough?

>
> Thanks,
>
> Lukas