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

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Date: Thu, 9 May 2024 15:33:13 +0100
From: Robin Murphy <robin.murphy@....com>
To: Steven Price <steven.price@....com>,
 Alexander Lobakin <aleksander.lobakin@...el.com>,
 Christoph Hellwig <hch@....de>
Cc: Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Marek Szyprowski <m.szyprowski@...sung.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, "Rafael J. Wysocki" <rafael@...nel.org>,
 Magnus Karlsson <magnus.karlsson@...el.com>,
 nex.sw.ncis.osdt.itp.upstreaming@...el.com, bpf@...r.kernel.org,
 netdev@...r.kernel.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/7] dma: avoid redundant calls for sync operations

On 09/05/2024 2:43 pm, Steven Price wrote:
> On 07/05/2024 12:20, 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@....de> # direct DMA shortcut
>> Co-developed-by: Eric Dumazet <edumazet@...gle.com>
>> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> 
> I've bisected a boot failure (on a Firefly RK3288) to this commit.
> AFAICT the problem is that I have (at least) two drivers which don't
> call dma_set_mask() and therefore never initialise the new dma_need_sync
> variable.
> 
> The specific drivers are "rockchip-drm" and "rk_gmac-dwmac". Is it a
> requirement that all drivers engaging in DMA should call dma_set_mask()
> - and therefore this has uncovered a bug in those drivers. Or is the
> assumption that all drivers call dma_set_mask() faulty?

Historically it's long been documented (at least in DMA-API-HOWTO) that 
a 32-bit DMA mask is assumed by default, so as much as we would prefer 
to shift expectations, there are still going to be a great many drivers 
relying on that :(

Perhaps its time for dma-debug to start warning about implicit mask 
usage, maybe that might help push the agenda a bit?

Thanks,
Robin.