[RFC PATCH v6 5/5] mmc: queue: Use bigger segments if IOMMU can merge the segments

[Yoshihiro Shimoda]

Hi Christoph,

> From: Christoph Hellwig, Sent: Friday, June 14, 2019 4:25 PM
> 
> On Thu, Jun 13, 2019 at 07:20:15PM +0900, Yoshihiro Shimoda wrote:
> > +static unsigned int mmc_get_max_segments(struct mmc_host *host)
> > +{
> > +	return host->can_merge ? BLK_MAX_SEGMENTS : host->max_segs;
> > +}
> 
> Note that BLK_MAX_SEGMENTS is really a little misnamed, it just
> is a BLK_DEFAULT_SEGMENTS.  I think you are better of picking your
> own value here (even if 128 ends up ok) than reusing this somewhat
> confusing constant.

Thank you for your comments. I got it. I'll fix this.

> > +	/*
> > +	 * Since blk_mq_alloc_tag_set() calls .init_request() of mmc_mq_ops,
> > +	 * the host->can_merge should be set before to get max_segs from
> > +	 * mmc_get_max_segments().
> > +	 */
> > +	if (host->max_segs < BLK_MAX_SEGMENTS &&
> > +	    device_iommu_mapped(mmc_dev(host)))
> > +		host->can_merge = 1;
> > +	else
> > +		host->can_merge = 0;
> > +
> 
> can_merge seems a little too generic a name to me.  Maybe can_iommu_merge?

I'll fix the name. Also, only the device_iommu_mapped() condition wiil cause
a problem on iommu=pt [1]. So, I'll add another condition here.

[1]
https://marc.info/?l=linux-mmc&m=156050608709643&w=2

Best regards,
Yoshihiro Shimoda