Re: [PATCH v6 2/7] dma: avoid redundant calls for sync operations

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Marek Szyprowski <m.szyprowski@samsung.com>
Date: Thu, 9 May 2024 13:41:16 +0200

> Dear All,
> 
> On 07.05.2024 13:20, Alexander Lobakin wrote:
>> Quite often, devices do not need dma_sync operations on x86_64 at least.
>> Indeed, when dev_is_dma_coherent(dev) is true and
>> dev_use_swiotlb(dev) is false, iommu_dma_sync_single_for_cpu()
>> and friends do nothing.
>>
>> However, indirectly calling them when CONFIG_RETPOLINE=y consumes about
>> 10% of cycles on a cpu receiving packets from softirq at ~100Gbit rate.
>> Even if/when CONFIG_RETPOLINE is not set, there is a cost of about 3%.
>>
>> Add dev->need_dma_sync boolean and turn it off during the device
>> initialization (dma_set_mask()) depending on the setup:
>> dev_is_dma_coherent() for the direct DMA, !(sync_single_for_device ||
>> sync_single_for_cpu) or the new dma_map_ops flag, %DMA_F_CAN_SKIP_SYNC,
>> advertised for non-NULL DMA ops.
>> Then later, if/when swiotlb is used for the first time, the flag
>> is reset back to on, from swiotlb_tbl_map_single().
>>
>> On iavf, the UDP trafficgen with XDP_DROP in skb mode test shows
>> +3-5% increase for direct DMA.
>>
>> Suggested-by: Christoph Hellwig <hch@lst.de> # direct DMA shortcut
>> Co-developed-by: Eric Dumazet <edumazet@google.com>
>> Signed-off-by: Eric Dumazet <edumazet@google.com>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
>> ---
>>   include/linux/device.h      |  4 +++
>>   include/linux/dma-map-ops.h | 12 ++++++++
>>   include/linux/dma-mapping.h | 53 +++++++++++++++++++++++++++++++----
>>   kernel/dma/mapping.c        | 55 +++++++++++++++++++++++++++++--------
>>   kernel/dma/swiotlb.c        |  6 ++++
>>   5 files changed, 113 insertions(+), 17 deletions(-)
> 
> 
> This patch landed in today's linux-next as commit f406c8e4b770 ("dma: 
> avoid redundant calls for sync operations"). Unfortunately I found that 
> it breaks some of the ARM 32bit boards by forcing skipping DMA sync 
> operations on non-coherent systems. This happens because this patch 
> hooks dma_need_sync=true initialization into set_dma_mask(), but 
> set_dma_mask() is not called from all device drivers, especially from 
> those which operates properly with the default 32bit dma mask (like most 
> of the platform devices created by the OF layer).
> 
> Frankly speaking I have no idea how this should be fixed. I expect that 
> there are lots of broken devices after this change, because I don't 
> remember that calling set_dma_mask() is mandatory for device drivers.
> 
> After adding dma_set_mask(dev, DMA_BIT_MASK(32)) to the drivers relevant 
> for my boards the issues are gone, but I'm not sure this is the right 
> approach...

If I remember correctly, *all* device drivers which use DMA *must* call
dma_set_*mask() on probe. That's why we added it there and didn't care.
Alternatively, if it really breaks a lot of drivers, we can set
dma_need_sync = true by default before the driver probing. I thought of
this, but the correct approach would be to call dma_set_*mask() from the
respective drivers.

> 
> 
>> ...
> 
> Best regards

Thanks,
Olek