linux-kernel - Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within dma_pool

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Date:   Tue, 16 Aug 2022 14:39:58 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Liu Song <liusong@...ux.alibaba.com>, akpm@...ux-foundation.org,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/dmapool.c: avoid duplicate memset within
 dma_pool_alloc

Hi,

On 18.07.2022 08:28, Liu Song wrote:
> From: Liu Song <liusong@...ux.alibaba.com>
>
> In "dma_alloc_from_dev_coherent" and "dma_direct_alloc",
> the allocated memory is explicitly set to 0.
>
> A helper function "use_dev_coherent_memory" is introduced here to
> determine whether the memory is allocated by "dma_alloc_from_dev_coherent".
>
> And use "get_dma_ops" to determine whether the memory is allocated by
> "dma_direct_alloc".
>
> After this modification, memory allocated using "dma_pool_zalloc" can avoid
> duplicate memset.
>
> Signed-off-by: Liu Song <liusong@...ux.alibaba.com>

This patch landed linux next-20220816. Unfortunately it causes serious 
issues on ARM 32bit systems. I've observed it on ARM 32bit Samsung 
Exynos 5422 based Odroid XU4 board with USB r8152 driver. After applying 
this patch and loading r8152 driver I only the following endless 
messages in the log:

xhci-hcd xhci-hcd.9.auto: WARN Event TRB for slot 1 ep 0 with no TDs queued?
xhci-hcd xhci-hcd.9.auto: WARN Event TRB for slot 1 ep 0 with no TDs queued?
xhci-hcd xhci-hcd.9.auto: WARN Event TRB for slot 1 ep 0 with no TDs queued?

It looks that there are drivers which rely on the fact that the dma 
coherent buffers are always zeroed.

> ---
>   include/linux/dma-map-ops.h | 5 +++++
>   mm/dmapool.c                | 5 ++++-
>   2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
> index 0d5b06b..c29948d 100644
> --- a/include/linux/dma-map-ops.h
> +++ b/include/linux/dma-map-ops.h
> @@ -171,6 +171,10 @@ int dma_alloc_from_dev_coherent(struct device *dev, ssize_t size,
>   int dma_release_from_dev_coherent(struct device *dev, int order, void *vaddr);
>   int dma_mmap_from_dev_coherent(struct device *dev, struct vm_area_struct *vma,
>   		void *cpu_addr, size_t size, int *ret);
> +static inline bool use_dev_coherent_memory(struct device *dev)
> +{
> +	return dev->dma_mem ? true : false;
> +}
>   #else
>   static inline int dma_declare_coherent_memory(struct device *dev,
>   		phys_addr_t phys_addr, dma_addr_t device_addr, size_t size)
> @@ -180,6 +184,7 @@ static inline int dma_declare_coherent_memory(struct device *dev,
>   #define dma_alloc_from_dev_coherent(dev, size, handle, ret) (0)
>   #define dma_release_from_dev_coherent(dev, order, vaddr) (0)
>   #define dma_mmap_from_dev_coherent(dev, vma, vaddr, order, ret) (0)
> +#define use_dev_coherent_memory(dev) (0)
>   #endif /* CONFIG_DMA_DECLARE_COHERENT */
>   
>   #ifdef CONFIG_DMA_GLOBAL_POOL
> diff --git a/mm/dmapool.c b/mm/dmapool.c
> index a7eb5d0..6e03530 100644
> --- a/mm/dmapool.c
> +++ b/mm/dmapool.c
> @@ -21,6 +21,7 @@
>   
>   #include <linux/device.h>
>   #include <linux/dma-mapping.h>
> +#include <linux/dma-map-ops.h>
>   #include <linux/dmapool.h>
>   #include <linux/kernel.h>
>   #include <linux/list.h>
> @@ -372,7 +373,9 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
>   #endif
>   	spin_unlock_irqrestore(&pool->lock, flags);
>   
> -	if (want_init_on_alloc(mem_flags))
> +	if (want_init_on_alloc(mem_flags) &&
> +		!use_dev_coherent_memory(pool->dev) &&
> +		get_dma_ops(pool->dev))
>   		memset(retval, 0, pool->size);
>   
>   	return retval;

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland