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From: Jason Gunthorpe <jgg@nvidia.com>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Maor Gottlieb <maorg@nvidia.com>, Christoph Hellwig <hch@lst.de>,
	"Gal Pressman" <galpress@amazon.com>,
	Bob Pearson <rpearsonhpe@gmail.com>,
	"Leon Romanovsky" <leonro@nvidia.com>,
	<linux-rdma@vger.kernel.org>
Subject: Re: dynamic-sg patch has broken rdma_rxe
Date: Tue, 20 Oct 2020 09:56:59 -0300
Message-ID: <20201020125659.GD6219@nvidia.com> (raw)
In-Reply-To: <c64bc07a-ed95-f462-a394-4191605c05b9@linux.intel.com>

On Tue, Oct 20, 2020 at 01:31:27PM +0100, Tvrtko Ursulin wrote:
> 
> On 20/10/2020 12:47, Jason Gunthorpe wrote:
> > On Tue, Oct 20, 2020 at 12:37:05PM +0100, Tvrtko Ursulin wrote:
> > 
> > > > Why put this confusing code in every caller? Especially for something
> > > > a driver is supposed to call. Will just make bugs
> > > 
> > > For max_segment to be aligned is a requirement today so callers are
> > > ready.
> > 
> > No, it turns out all the RDMA drivers were became broken when they
> > converted to use the proper U32_MAX for their DMA max_segment size,
> > then they couldn't form SGLs anymore.
> > 
> > I don't want to see nonsense code like this:
> > 
> >          dma_set_max_seg_size(dev->dev, min_t(unsigned int, U32_MAX & PAGE_MASK,
> >                                               SCATTERLIST_MAX_SEGMENT));
> > 
> > In drivers.
> > 
> > dma_set_max_seg_size is the *hardware* capability, and mixing in
> > things like PAG_MASK here is just nonsense.
> 
> Code was obviously a no-op non-sense.
> 
> So the crux of the argument is that U32_MAX is conceptually the right thing
> which defines the DMA max_segment size? Not some DMA define or anything, but
> really U32_MAX? And that all/some DMA hardware does not think in pages but
> really in bytes?

Yes.

The HW has 32 bits for a length field, so U32_MAX accurately defines
the HW DMA capability

Jason

  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-13 14:33 Bob Pearson
2020-10-13 16:34 ` Bob Pearson
2020-10-14 22:51 ` Jason Gunthorpe
2020-10-15  7:44   ` Maor Gottlieb
2020-10-15 11:23     ` Gal Pressman
2020-10-15 12:21       ` Maor Gottlieb
2020-10-16  0:31         ` Jason Gunthorpe
2020-10-16  7:13           ` Christoph Hellwig
     [not found]           ` <796ca31aed8f469c957cb850385b9d09@intel.com>
2020-10-16 11:58             ` Jason Gunthorpe
2020-10-19  9:50               ` Tvrtko Ursulin
2020-10-19 12:12                 ` Jason Gunthorpe
2020-10-19 12:29                   ` Tvrtko Ursulin
2020-10-19 12:48                     ` Jason Gunthorpe
2020-10-20 11:37                       ` Tvrtko Ursulin
2020-10-20 11:47                         ` Jason Gunthorpe
2020-10-20 12:31                           ` Tvrtko Ursulin
2020-10-20 12:56                             ` Jason Gunthorpe [this message]
2020-10-20 13:09                               ` Tvrtko Ursulin
2020-10-20 13:32                                 ` Jason Gunthorpe
2020-10-15 15:35     ` Bob Pearson

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