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From:   "hch@lst.de" <hch@lst.de>
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>
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        Tianyu Lan <ltykernel@gmail.com>,
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Subject: Re: [PATCH] swiotlb: Max mapping size takes min align mask into
 account
Message-ID: <20220511060212.GA32192@lst.de>
References: <20220510142109.777738-1-ltykernel@gmail.com> <cd64adcd-26fc-0452-754d-7ab0f5536142@arm.com> <PH0PR21MB302530B081D6CE6470D5F9A4D7C99@PH0PR21MB3025.namprd21.prod.outlook.com>
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On Tue, May 10, 2022 at 06:26:55PM +0000, Michael Kelley (LINUX) wrote:
> > Hmm, this seems a bit pessimistic - the offset can vary per mapping, so
> > it feels to me like it should really be the caller's responsibility to
> > account for it if they're already involved enough to care about both
> > constraints. But I'm not sure how practical that would be.
> 
> Tianyu and I discussed this prior to his submitting the patch.
> Presumably dma_max_mapping_size() exists so that the higher
> level blk-mq code can limit the size of I/O requests to something
> that will "fit" in the swiotlb when bounce buffering is enabled.

Yes, the idea that upper level code doesn't need to care was very
much the idea behind dma_max_mapping_size().

> As you mentioned, how else would a caller handle this situation?

Well, we could look at dma_get_min_align_mask in the caller and do
the calculation there, but I really don't think that is a good idea.

So this patch looks sensible to me.