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

Steven Price
2024-05-09T13:43:59Z

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@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>

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?

Thanks,

Steve

> ---
> 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(-)
>
> diff --git a/include/linux/device.h b/include/linux/device.h
> index b9f5464f44ed..ed95b829f05b 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -691,6 +691,7 @@ struct device_physical_location {
> * and optionall (if the coherent mask is large enough) also
> * for dma allocations. This flag is managed by the dma ops
> * instance from ->dma_supported.
> + * @dma_need_sync: The device needs performing DMA sync operations.
> *
> * At the lowest level, every device in a Linux system is represented by an
> * instance of struct device. The device structure contains the information
> @@ -803,6 +804,9 @@ struct device {
> #ifdef CONFIG_DMA_OPS_BYPASS
> bool dma_ops_bypass : 1;
> #endif
> +#ifdef CONFIG_DMA_NEED_SYNC
> + bool dma_need_sync:1;
> +#endif
> };
>
> /**
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 4abc60f04209..4893cb89cb52 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -18,8 +18,11 @@ struct iommu_ops;
> *
> * DMA_F_PCI_P2PDMA_SUPPORTED: Indicates the dma_map_ops implementation can
> * handle PCI P2PDMA pages in the map_sg/unmap_sg operation.
> + * DMA_F_CAN_SKIP_SYNC: DMA sync operations can be skipped if the device is
> + * coherent and it's not an SWIOTLB buffer.
> */
> #define DMA_F_PCI_P2PDMA_SUPPORTED (1 << 0)
> +#define DMA_F_CAN_SKIP_SYNC (1 << 1)
>
> struct dma_map_ops {
> unsigned int flags;
> @@ -273,6 +276,15 @@ static inline bool dev_is_dma_coherent(struct device *dev)
> }
> #endif /* CONFIG_ARCH_HAS_DMA_COHERENCE_H */
>
> +static inline void dma_reset_need_sync(struct device *dev)
> +{
> +#ifdef CONFIG_DMA_NEED_SYNC
> + /* Reset it only once so that the function can be called on hotpath */
> + if (unlikely(!dev->dma_need_sync))
> + dev->dma_need_sync = true;
> +#endif
> +}
> +
> /*
> * Check whether potential kmalloc() buffers are safe for non-coherent DMA.
> */
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index a569b56b25e2..eb4e15893b6c 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -282,16 +282,59 @@ static inline int dma_mmap_noncontiguous(struct device *dev,
> #endif /* CONFIG_HAS_DMA */
>
> #if defined(CONFIG_HAS_DMA) && defined(CONFIG_DMA_NEED_SYNC)
> -void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr, size_t size,
> +void __dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr, size_t size,
> enum dma_data_direction dir);
> -void dma_sync_single_for_device(struct device *dev, dma_addr_t addr,
> +void __dma_sync_single_for_device(struct device *dev, dma_addr_t addr,
> size_t size, enum dma_data_direction dir);
> -void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
> +void __dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir);
> -void dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
> +void __dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir);
> -bool dma_need_sync(struct device *dev, dma_addr_t dma_addr);
> +bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr);
> +
> +static inline bool dma_dev_need_sync(const struct device *dev)
> +{
> + /* Always call DMA sync operations when debugging is enabled */
> + return dev->dma_need_sync || IS_ENABLED(CONFIG_DMA_API_DEBUG);
> +}
> +
> +static inline void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
> + size_t size, enum dma_data_direction dir)
> +{
> + if (dma_dev_need_sync(dev))
> + __dma_sync_single_for_cpu(dev, addr, size, dir);
> +}
> +
> +static inline void dma_sync_single_for_device(struct device *dev,
> + dma_addr_t addr, size_t size, enum dma_data_direction dir)
> +{
> + if (dma_dev_need_sync(dev))
> + __dma_sync_single_for_device(dev, addr, size, dir);
> +}
> +
> +static inline void dma_sync_sg_for_cpu(struct device *dev,
> + struct scatterlist *sg, int nelems, enum dma_data_direction dir)
> +{
> + if (dma_dev_need_sync(dev))
> + __dma_sync_sg_for_cpu(dev, sg, nelems, dir);
> +}
> +
> +static inline void dma_sync_sg_for_device(struct device *dev,
> + struct scatterlist *sg, int nelems, enum dma_data_direction dir)
> +{
> + if (dma_dev_need_sync(dev))
> + __dma_sync_sg_for_device(dev, sg, nelems, dir);
> +}
> +
> +static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> +{
> + return dma_dev_need_sync(dev) ? __dma_need_sync(dev, dma_addr) : false;
> +}
> #else /* !CONFIG_HAS_DMA || !CONFIG_DMA_NEED_SYNC */
> +static inline bool dma_dev_need_sync(const struct device *dev)
> +{
> + return false;
> +}
> static inline void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
> size_t size, enum dma_data_direction dir)
> {
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index c78b78e95a26..3524bc92c37f 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -330,7 +330,7 @@ void dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size,
> EXPORT_SYMBOL(dma_unmap_resource);
>
> #ifdef CONFIG_DMA_NEED_SYNC
> -void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr, size_t size,
> +void __dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr, size_t size,
> enum dma_data_direction dir)
> {
> const struct dma_map_ops *ops = get_dma_ops(dev);
> @@ -342,9 +342,9 @@ void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr, size_t size,
> ops->sync_single_for_cpu(dev, addr, size, dir);
> debug_dma_sync_single_for_cpu(dev, addr, size, dir);
> }
> -EXPORT_SYMBOL(dma_sync_single_for_cpu);
> +EXPORT_SYMBOL(__dma_sync_single_for_cpu);
>
> -void dma_sync_single_for_device(struct device *dev, dma_addr_t addr,
> +void __dma_sync_single_for_device(struct device *dev, dma_addr_t addr,
> size_t size, enum dma_data_direction dir)
> {
> const struct dma_map_ops *ops = get_dma_ops(dev);
> @@ -356,9 +356,9 @@ void dma_sync_single_for_device(struct device *dev, dma_addr_t addr,
> ops->sync_single_for_device(dev, addr, size, dir);
> debug_dma_sync_single_for_device(dev, addr, size, dir);
> }
> -EXPORT_SYMBOL(dma_sync_single_for_device);
> +EXPORT_SYMBOL(__dma_sync_single_for_device);
>
> -void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
> +void __dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir)
> {
> const struct dma_map_ops *ops = get_dma_ops(dev);
> @@ -370,9 +370,9 @@ void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg,
> ops->sync_sg_for_cpu(dev, sg, nelems, dir);
> debug_dma_sync_sg_for_cpu(dev, sg, nelems, dir);
> }
> -EXPORT_SYMBOL(dma_sync_sg_for_cpu);
> +EXPORT_SYMBOL(__dma_sync_sg_for_cpu);
>
> -void dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
> +void __dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
> int nelems, enum dma_data_direction dir)
> {
> const struct dma_map_ops *ops = get_dma_ops(dev);
> @@ -384,18 +384,47 @@ void dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg,
> ops->sync_sg_for_device(dev, sg, nelems, dir);
> debug_dma_sync_sg_for_device(dev, sg, nelems, dir);
> }
> -EXPORT_SYMBOL(dma_sync_sg_for_device);
> +EXPORT_SYMBOL(__dma_sync_sg_for_device);
>
> -bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> +bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> {
> const struct dma_map_ops *ops = get_dma_ops(dev);
>
> if (dma_map_direct(dev, ops))
> + /*
> + * dma_need_sync could've been reset on first SWIOTLB buffer
> + * mapping, but @dma_addr is not necessary an SWIOTLB buffer.
> + * In this case, fall back to more granular check.
> + */
> return dma_direct_need_sync(dev, dma_addr);
> - return ops->sync_single_for_cpu || ops->sync_single_for_device;
> + return true;
> }
> -EXPORT_SYMBOL_GPL(dma_need_sync);
> -#endif /* CONFIG_DMA_NEED_SYNC */
> +EXPORT_SYMBOL_GPL(__dma_need_sync);
> +
> +static void dma_setup_need_sync(struct device *dev)
> +{
> + const struct dma_map_ops *ops = get_dma_ops(dev);
> +
> + if (dma_map_direct(dev, ops) || (ops->flags & DMA_F_CAN_SKIP_SYNC))
> + /*
> + * dma_need_sync will be reset to %true on first SWIOTLB buffer
> + * mapping, if any. During the device initialization, it's
> + * enough to check only for the DMA coherence.
> + */
> + dev->dma_need_sync = !dev_is_dma_coherent(dev);
> + else if (!ops->sync_single_for_device && !ops->sync_single_for_cpu &&
> + !ops->sync_sg_for_device && !ops->sync_sg_for_cpu)
> + /*
> + * Synchronization is not possible when none of DMA sync ops
> + * is set.
> + */
> + dev->dma_need_sync = false;
> + else
> + dev->dma_need_sync = true;
> +}
> +#else /* !CONFIG_DMA_NEED_SYNC */
> +static inline void dma_setup_need_sync(struct device *dev) { }
> +#endif /* !CONFIG_DMA_NEED_SYNC */
>
> /*
> * The whole dma_get_sgtable() idea is fundamentally unsafe - it seems
> @@ -785,6 +814,8 @@ int dma_set_mask(struct device *dev, u64 mask)
>
> arch_dma_set_mask(dev, mask);
> *dev->dma_mask = mask;
> + dma_setup_need_sync(dev);
> +
> return 0;
> }
> EXPORT_SYMBOL(dma_set_mask);
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 046da973a7e2..ae3e593eaadb 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -1408,6 +1408,12 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
> return (phys_addr_t)DMA_MAPPING_ERROR;
> }
>
> + /*
> + * If dma_need_sync wasn't set, reset it on first SWIOTLB buffer
> + * mapping to always sync SWIOTLB buffers.
> + */
> + dma_reset_need_sync(dev);
> +
> /*
> * Save away the mapping from the original address to the DMA address.
> * This is needed when we sync the memory. Then we sync the buffer if