linux-kernel - Re: [PATCH 1/3] Adding page_offset_mask to device_dma

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Date:   Thu, 28 Jan 2021 19:15:57 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Jianxiong Gao <jxgao@...gle.com>, erdemaktas@...gle.com,
        marcorr@...gle.com, hch@....de, m.szyprowski@...sung.com,
        gregkh@...uxfoundation.org, saravanak@...gle.com,
        heikki.krogerus@...ux.intel.com, rafael.j.wysocki@...el.com,
        andriy.shevchenko@...ux.intel.com, dan.j.williams@...el.com,
        bgolaszewski@...libre.com, jroedel@...e.de,
        iommu@...ts.linux-foundation.org, konrad.wilk@...cle.com,
        kbusch@...nel.org, axboe@...com, sagi@...mberg.me,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] Adding page_offset_mask to device_dma_parameters

On Thu, Jan 28, 2021 at 05:27:25PM +0000, Robin Murphy wrote:
> On 2021-01-28 00:38, Jianxiong Gao wrote:
>> Some devices rely on the address offset in a page to function
>> correctly (NVMe driver as an example). These devices may use
>> a different page size than the Linux kernel. The address offset
>> has to be preserved upon mapping, and in order to do so, we
>> need to record the page_offset_mask first.
>>
>> Signed-off-by: Jianxiong Gao <jxgao@...gle.com>
>> ---
>>   include/linux/device.h      |  1 +
>>   include/linux/dma-mapping.h | 17 +++++++++++++++++
>>   2 files changed, 18 insertions(+)
>>
>> diff --git a/include/linux/device.h b/include/linux/device.h
>> index 1779f90eeb4c..f44e0659fc66 100644
>> --- a/include/linux/device.h
>> +++ b/include/linux/device.h
>> @@ -292,6 +292,7 @@ struct device_dma_parameters {
>>   	 */
>>   	unsigned int max_segment_size;
>>   	unsigned long segment_boundary_mask;
>> +	unsigned int page_offset_mask;
>
> Could we call this something more like "min_align_mask" (sorry, I can't 
> think of a name that's actually good and descriptive right now). 
> Essentially I worry that having "page" in there is going to be too easy to 
> misinterpret as having anything to do what "page" means almost everywhere 
> else (even before you throw IOMMU pages into the mix).
>
> Also note that of all the possible ways to pack two ints and a long, this 
> one is the worst ;)

The block layer uses virt_boundary for the related concept, but that
is pretty horrible too.