RE: [RFC PATCH v6 4/5] mmc: tmio: Use dma_max_mapping_size() instead of a workaround

[Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>]

Hi Geert, Christoph,

Thank you for your comments!

> From: Geert Uytterhoeven, Sent: Friday, June 14, 2019 4:27 PM
> 
> Hi Christoph,
> 
> On Fri, Jun 14, 2019 at 9:18 AM Christoph Hellwig wrote:
> > On Thu, Jun 13, 2019 at 10:35:44PM +0200, Geert Uytterhoeven wrote:
> > > I'm always triggered by the use of min_t() and other casts:
> > > mmc->max_blk_size and mmc->max_blk_count are both unsigned int.
> > > dma_max_mapping_size() returns size_t, which can be 64-bit.
> > >
> > >  1) Can the multiplication overflow?
> > >     Probably not, as per commit 2a55c1eac7882232 ("mmc: renesas_sdhi:
> > >     prevent overflow for max_req_size"), but I thought I'd better ask.

Geert-san:

I agree.

> > >  2) In theory, dma_max_mapping_size() can return a number that doesn't
> > >     fit in 32-bit, and will be truncated (to e.g. 0), leading to max_req_size
> > >     is zero?

Geert-san:

I agree. If dma_max_mapping_size() return 0x1_0000_0000, it will be truncated to 0
and then max_req_size is set to zero. It is a problem. Also, the second argument
"mmc->max_blk_size * mmc->max_blk_count" will not be overflow and then the value is
0xffff_ffff or less. So, I also think this should use size_t instead of unsigned int.

> > This really should use a min_t on size_t.  Otherwise the patch looks
> > fine:
> 
> Followed by another min() to make it fit in mmc->max_req_size, which is
> unsigned int.

Geert-san:

I'm afraid, but I cannot understand this means.
Is this patch is possible to be upstream? Or, do you have any concern?


Best regards,
Yoshihiro Shimoda