Re: [PATCH v5 4/5] scsi: scsi_transport_sas: Cap shost max_sectors according to DMA optimal limit

[John Garry]

On 01/07/2022 00:49, Damien Le Moal wrote:
> >   
> > +	if (dma_dev) {
> > +		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
> > +				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
> > +	}

Hi Damien,

> Hmm... shost->max_sectors becomes the max_hw_sectors limit for the block
> dev. So using dma_max_mapping_size(dma_dev) for that limit makes sense.
> Shouldn't dma_opt_mapping_size(dma_dev) be used to limit only the default
> "soft" limit (queue max_sectors limit) instead of the hard limit ?
>

Sure, it would sensible to use dma_opt_mapping_size() to limit the 
default queue max sectors limit, while dma_max_mapping_size() limits the 
host max sectors. But I didn't see in practice how limiting the shost 
max sectors to dma_opt_mapping_size() makes a difference:

- block queue max_hw_sectors_kb file is read-only, so we cannot change 
the queue max sectors from there

- And no SAS driver actually tries to modify upwards from the default.
I do note that USB storage driver as an example of a scsi driver which 
does (modify from shost max sectors): see scsiglue.c::slave_configure()

Finally there is no common method to limit the default request queue max 
sectors for those SAS drivers - I would need to add this limit in each 
of their slave_configure callbacks, and I didn't think that its worth it.

Thanks,
John