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SubjectRe: [PATCH 02/20] dma-mapping: provide a generic dma-noncoherent implementation
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Hi Christoph,

On Fri, 2018-05-11 at 09:59 +0200, Christoph Hellwig wrote:

[snip]

There seems to be one subtle issue with map/unmap code.
While investigating problems on ARC I added instrumentation as below:
---------------------------------------->8------------------------------------
--- a/arch/arc/mm/dma.c
+++ b/arch/arc/mm/dma.c
@@ -152,14 +152,37 @@ static void _dma_cache_sync(struct device *dev, phys_addr_t paddr, size_t size,
}
}

+static const char *dir_to_str(enum dma_data_direction dir)
+{
+ switch (dir) {
+ case DMA_BIDIRECTIONAL: return "DMA_BIDIRECTIONAL";
+ case DMA_TO_DEVICE: return "DMA_TO_DEVICE";
+ case DMA_FROM_DEVICE: return "DMA_FROM_DEVICE";
+ case DMA_NONE: return "DMA_NONE";
+ default: return "WRONG_VALUE!";
+ }
+}
+
void arch_sync_dma_for_device(struct device *dev, phys_addr_t paddr,
size_t size, enum dma_data_direction dir)
{
+ if (dir != DMA_TO_DEVICE){
+ dump_stack();
+ printk(" *** %s@%d: DMA direction is %s instead of %s\n",
+ __func__, __LINE__, dir_to_str(dir), dir_to_str(DMA_TO_DEVICE));
+ }
+
return _dma_cache_sync(dev, paddr, size, dir);
}

void arch_sync_dma_for_cpu(struct device *dev, phys_addr_t paddr,
size_t size, enum dma_data_direction dir)
{
+ if (dir != DMA_FROM_DEVICE) {
+ dump_stack();
+ printk(" *** %s@%d: DMA direction is %s instead of %s\n",
+ __func__, __LINE__, dir_to_str(dir), dir_to_str(DMA_FROM_DEVICE));
+ }
+
return _dma_cache_sync(dev, paddr, size, dir);
}
---------------------------------------->8------------------------------------
And with that I noticed a bit unexpected output, see below:
---------------------------------------->8------------------------------------
Stack Trace:
arc_unwind_core.constprop.1+0xd4/0xf8
dump_stack+0x68/0x80
arch_sync_dma_for_device+0x34/0xc4
dma_noncoherent_map_sg+0x80/0x94
__dw_mci_start_request+0x1ee/0x868
dw_mci_request+0x17e/0x1c8
mmc_wait_for_req+0x106/0x1ac
mmc_app_sd_status+0x108/0x130
mmc_sd_setup_card+0xc6/0x2e8
mmc_attach_sd+0x1b6/0x394
mmc_rescan+0x2f4/0x3bc
process_one_work+0x194/0x348
worker_thread+0xf2/0x478
kthread+0x120/0x13c
ret_from_fork+0x18/0x1c
*** arch_sync_dma_for_device@172: DMA direction is DMA_FROM_DEVICE instead of DMA_TO_DEVICE
...
Stack Trace:
arc_unwind_core.constprop.1+0xd4/0xf8
dump_stack+0x68/0x80
arch_sync_dma_for_device+0x34/0xc4
dma_noncoherent_map_page+0x86/0x8c
usb_hcd_map_urb_for_dma+0x49e/0x53c
usb_hcd_submit_urb+0x43c/0x8c4
usb_control_msg+0xbe/0x16c
hub_port_init+0x5e0/0xb0c
hub_event+0x4e6/0x1164
process_one_work+0x194/0x348
worker_thread+0xf2/0x478
kthread+0x120/0x13c
ret_from_fork+0x18/0x1c
mmcblk0: p1 p2
*** arch_sync_dma_for_device@172: DMA direction is DMA_FROM_DEVICE instead of DMA_TO_DEVICE
...
and quite some more of the similar
...
---------------------------------------->8------------------------------------
In case of MMC/DW_MCI (AKA DesignWare MobileStorage controller) that's an execution flow:
1) __dw_mci_start_request()
2) dw_mci_pre_dma_transfer()
3) dma_map_sg(..., mmc_get_dma_dir(data))

Note mmc_get_dma_dir() is just "data->flags & MMC_DATA_WRITE ? DMA_TO_DEVICE : DMA_FROM_DEVICE".
I.e. if we're preparing for sending data dma_noncoherent_map_sg() will have DMA_TO_DEVICE which
is quite OK for passing to dma_noncoherent_sync_sg_for_device() but in case of reading we'll have
DMA_FROM_DEVICE which we'll pass to dma_noncoherent_sync_sg_for_device() in dma_noncoherent_map_sg().

I'd say this is not entirely correct because IMHO arch_sync_dma_for_cpu() is supposed to only be used
in case of DMA_FROM_DEVICE and arch_sync_dma_for_device() only in case of DMA_TO_DEVICE.


> +static dma_addr_t dma_noncoherent_map_page(struct device *dev, struct page *page,
> + unsigned long offset, size_t size, enum dma_data_direction dir,
> + unsigned long attrs)
> +{
> + dma_addr_t addr;
> +
> + addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
> + if (!dma_mapping_error(dev, addr) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> + arch_sync_dma_for_device(dev, page_to_phys(page), size, dir);
> + return addr;
> +}
> +
> +static int dma_noncoherent_map_sg(struct device *dev, struct scatterlist *sgl,
> + int nents, enum dma_data_direction dir, unsigned long attrs)
> +{
> + nents = dma_direct_map_sg(dev, sgl, nents, dir, attrs);
> + if (nents > 0 && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> + dma_noncoherent_sync_sg_for_device(dev, sgl, nents, dir);
> + return nents;
> +}

The same is for unmap functions.
My guess is we need to respect direction in map/unmap functions and use
either dma_noncoherent_sync_single_for_cpu(..., DMA_FROM_DEVICE) or
dma_noncoherent_sync_single_for_device(...,DMA_TO_DEVICE).


> +static void dma_noncoherent_unmap_page(struct device *dev, dma_addr_t addr,
> + size_t size, enum dma_data_direction dir, unsigned long attrs)
> +{
> + if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> + dma_noncoherent_sync_single_for_cpu(dev, addr, size, dir);
> +}
> +
> +static void dma_noncoherent_unmap_sg(struct device *dev, struct scatterlist *sgl,
> + int nents, enum dma_data_direction dir, unsigned long attrs)
> +{
> + if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> + dma_noncoherent_sync_sg_for_cpu(dev, sgl, nents, dir);
> +}
> +#endif

But the real fix of my problem is:
---------------------------------------->8------------------------------------
--- a/lib/dma-noncoherent.c
+++ b/lib/dma-noncoherent.c
@@ -35,7 +35,7 @@ static dma_addr_t dma_noncoherent_map_page(struct device *dev, struct page *page

addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
if (!dma_mapping_error(dev, addr) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
- arch_sync_dma_for_device(dev, page_to_phys(page), size, dir);
+ arch_sync_dma_for_device(dev, page_to_phys(page) + offset, size, dir);
return addr;
}
---------------------------------------->8------------------------------------
You seem to lost an offset in the page so if we happen to have a buffer not aligned to
a page boundary then we were obviously corrupting data outside our data :)

-Alexey
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