linux-kernel - Re: [PATCH v2 6/7] iommu/rockchip: use DMA API to map, to flush cache

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Date:	Fri, 10 Jun 2016 18:10:29 +0900
From:	Tomasz Figa <tfiga@...gle.com>
To:	Shunqian Zheng <zhengsq@...k-chips.com>
Cc:	Joerg Roedel <joro@...tes.org>,
	Heiko Stübner <heiko@...ech.de>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>, linux@...linux.org.uk,
	姚智情 <mark.yao@...k-chips.com>,
	David Airlie <airlied@...ux.ie>,
	simon xue <xxm@...k-chips.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
	devicetree@...r.kernel.org,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>
Subject: Re: [PATCH v2 6/7] iommu/rockchip: use DMA API to map, to flush cache

Hi,

On Wed, Jun 8, 2016 at 10:26 PM, Shunqian Zheng <zhengsq@...k-chips.com> wrote:
> Use DMA API instead of architecture internal functions like
> __cpuc_flush_dcache_area() etc.
>
> To support the virtual device like DRM the virtual slave iommu
> added in the previous patch, attaching to which the DRM can use
> it own domain->dev for dma_map_*(), dma_sync_*() even VOP is disabled.
>
> With this patch, this driver is available for ARM64 like RK3399.
>

Could we instead simply allocate coherent memory for page tables using
dma_alloc_coherent() and skip any flushing on CPU side completely? If
I'm looking correctly, the driver only reads back the page directory
when checking if there is a need to allocate new page table, so there
shouldn't be any significant penalty for disabling the cache.

Other than that, please see some comments inline.

> Signed-off-by: Shunqian Zheng <zhengsq@...k-chips.com>
> ---
>  drivers/iommu/rockchip-iommu.c | 113 ++++++++++++++++++++++++++---------------
>  1 file changed, 71 insertions(+), 42 deletions(-)
>
> diff --git a/drivers/iommu/rockchip-iommu.c b/drivers/iommu/rockchip-iommu.c
> index d6c3051..aafea6e 100644
> --- a/drivers/iommu/rockchip-iommu.c
> +++ b/drivers/iommu/rockchip-iommu.c
> @@ -4,8 +4,6 @@
>   * published by the Free Software Foundation.
>   */
>
> -#include <asm/cacheflush.h>
> -#include <asm/pgtable.h>
>  #include <linux/compiler.h>
>  #include <linux/delay.h>
>  #include <linux/device.h>
> @@ -61,8 +59,7 @@
>  #define RK_MMU_IRQ_BUS_ERROR     0x02  /* bus read error */
>  #define RK_MMU_IRQ_MASK          (RK_MMU_IRQ_PAGE_FAULT | RK_MMU_IRQ_BUS_ERROR)
>
> -#define NUM_DT_ENTRIES 1024
> -#define NUM_PT_ENTRIES 1024
> +#define NUM_TLB_ENTRIES 1024 /* for both DT and PT */

Is it necessary to change this in this patch? In general, it's not a
good idea to mix multiple logical changes together.

>
>  #define SPAGE_ORDER 12
>  #define SPAGE_SIZE (1 << SPAGE_ORDER)
> @@ -82,7 +79,9 @@
>
>  struct rk_iommu_domain {
>         struct list_head iommus;
> +       struct device *dev;
>         u32 *dt; /* page directory table */
> +       dma_addr_t dt_dma;
>         spinlock_t iommus_lock; /* lock for iommus list */
>         spinlock_t dt_lock; /* lock for modifying page directory table */
>
> @@ -98,14 +97,12 @@ struct rk_iommu {
>         struct iommu_domain *domain; /* domain to which iommu is attached */
>  };
>
> -static inline void rk_table_flush(u32 *va, unsigned int count)
> +static inline void rk_table_flush(struct device *dev, dma_addr_t dma,
> +                                 unsigned int count)
>  {
> -       phys_addr_t pa_start = virt_to_phys(va);
> -       phys_addr_t pa_end = virt_to_phys(va + count);
> -       size_t size = pa_end - pa_start;
> +       size_t size = count * 4;

It would be a good idea to specify what "count" is. I'm a bit confused
that before it meant bytes and now some multiple of 4?

Best regards,
Tomasz