linux-kernel - Re: [PATCH] tty: amba-pl011: use sg

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Date:	Tue, 8 Mar 2016 09:59:07 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Chanho Min <chanho.min@....com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jongsung Kim <neidhard.kim@....com>,
	Gunho Lee <gunho.lee@....com>
Subject: Re: [PATCH] tty: amba-pl011: use sg_table instead of scatterlist

On Tue, Mar 08, 2016 at 11:43:36AM +0900, Chanho Min wrote:
> The use of phys_to_page is wrong in commit cb06ff102e2d ("ARM: PL011:
> Add support for Rx DMA buffer polling."), beacase DMA address is not
> physical address. Also, phys_to_page is not declared in some architecture.
> The use of virt_to_page is wrong as well because dma_alloc_coherent
> implementaion are allowed to returned remapped memory. So I use sg_table
> instead of scatterlist and change to dma_get_sgtable() implementaion.

dma_get_sgtable() is also broken.

> @@ -344,17 +344,23 @@ static int pl011_sgbuf_init(struct dma_chan *chan, struct pl011_sgbuf *sg,
>  	enum dma_data_direction dir)
>  {
>  	dma_addr_t dma_addr;
> +	int ret;
>  
>  	sg->buf = dma_alloc_coherent(chan->device->dev,
>  		PL011_DMA_BUFFER_SIZE, &dma_addr, GFP_KERNEL);
>  	if (!sg->buf)
>  		return -ENOMEM;

sg->buf can be a mapped address, which virt_to_page() will return invalid
results:

int dma_common_get_sgtable(struct device *dev, struct sg_table *sgt,
                 void *cpu_addr, dma_addr_t handle, size_t size)
{
        struct page *page = virt_to_page(cpu_addr);
        int ret;

        ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
        if (unlikely(ret))
                return ret;

        sg_set_page(sgt->sgl, page, PAGE_ALIGN(size), 0);
        return 0;
}
EXPORT_SYMBOL(dma_common_get_sgtable);

I've no idea who is shoveling crap like this into the kernel, but it's
_really_ far from good that such broken abstractions are being merged
as generic code.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
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