Re: [PATCH RFC] net/mlx4: Remove improper usage of dma_alloc_coherent().

[Ido Shamay]

On 4/7/2015 10:43 PM, Ido Shamay wrote:
> On 4/7/2015 2:00 AM, David Daney wrote:
> > From: David Daney <david.daney@xxxxxxxxxx>
> >
> > The dma_alloc_coherent() function returns a virtual address which can
> > be used for coherent access to the underlying memory.  On some
> > architectures, like arm64, undefined behavior results if this memory is
> > also accessed via virtual mappings that are not coherent. Because of
> > their undefined nature, operations like virt_to_page() return garbage
> > when passed virtual addresses obtained from dma_alloc_coherent().  Any
> > subsequent mappings via vmap() of the garbage page values are unusable
> > and result in bad things like bus errors (synchronous aborts in ARM64
> > speak).
> >
> > The MLX4 driver contains code that does the equivalent of:
> >
> >    vmap(virt_to_page(dma_alloc_coherent))
> >
> > This results in an OOPs when the device is opened.
> >
> > To fix this...
> >
> > Always use result of dma_alloc_coherent() directly.
> Hi David,
>
> I'm not sure this solution is good enough for the common case(s).
> Typical allocation size will be around 64KB (with default 1K ring size).
> We can't rely on the system to always provide us with that amount of 
> contiguous memory.
>
> Current code allocation scheme is more robust, max_direct is typically 
> 2 * PAGE_SIZE,
> so pages from order 1 are far more available then higher order.
>
> I need to check why the code is written as it is today,  and not as in 
> this RFC (which is much more trivial).
> I'll continue to investigate tomorrow, will get back with some answers.
>
> Ido

Acked-by: Ido Shamay <idos@xxxxxxxxxxxx>
Thanks David, this is good for us

> > Remove 'max_direct' parameter to mlx4_buf_alloc(), as it is unused,
> > and adjust all callers.
> >
> > Remove mlx4_en_map_buffer() and mlx4_en_unmap_buffer() as they now do
> > nothing, and adjust all callers.
> >
> > Remove 'page_list' element from struct mlx4_buf as it is unused.
> >
> > Signed-off-by: David Daney <david.daney@xxxxxxxxxx>

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