Re: [PATCH 1/3] Adding page_offset_mask to device_dma_parameters

Christoph Hellwig
2021-01-28T18:20:05Z

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@google.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.