Re: [PATCH 1/2] dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

[Christoph Hellwig <hch@lst.de>]

On Mon, Jun 29, 2026 at 12:09:42PM +0100, Robin Murphy wrote:
>>  > Additionally, could this cause invalid zero-sized block layer queue 
>> limits
>>  > in MMC drivers? In drivers/mmc/host/bcm2835.c:bcm2835_add_host(), the
>>  > return value clamps max_req_size:
>>  >
>>  >          mmc->max_req_size = min_t(size_t, 524288, 
>> dma_max_mapping_size(dev));
>>
>> bcm2835.c is a platform device driver, and platform devices have their 
>> dev->dma_mask set in setup_pdev_dma_masks()
>>
>> Indeed, that driver does have a non-DMA mode of operation, but that looks 
>> to be selected independent of whether dev->dma_mask is set.
>
> That one seems pretty bogus already, given that DMA mode is apparently 
> dependent on an external DMA channel, so "dev" is the wrong device to check 
> (should be dmaengine_get_dma_device()), while conversely 
> dma_max_mapping_size(anything) is a questionably meaningless number for PIO 
> mode... :/

Yeah.  It would be good to get this fixed before this change hits mainline,
though.