Re: [RFC PATCH V4 1/1] swiotlb: Split up single swiotlb lock

[Tianyu Lan]

On 6/22/2022 6:54 PM, Christoph Hellwig wrote:
> Thanks,
>
> this looks pretty good to me.  A few comments below:

Thanks for your review.

> On Fri, Jun 17, 2022 at 10:47:41AM -0400, Tianyu Lan wrote:
> > +/**
> > + * struct io_tlb_area - IO TLB memory area descriptor
> > + *
> > + * This is a single area with a single lock.
> > + *
> > + * @used:	The number of used IO TLB block.
> > + * @index:	The slot index to start searching in this area for next round.
> > + * @lock:	The lock to protect the above data structures in the map and
> > + *		unmap calls.
> > + */
> > +struct io_tlb_area {
> > +	unsigned long used;
> > +	unsigned int index;
> > +	spinlock_t lock;
> > +};
>
> This can go into swiotlb.c.

struct io_tlb_area is used in the struct io_tlb_mem.

> > +void __init swiotlb_adjust_nareas(unsigned int nareas);
>
> And this should be marked static.
>
> > +#define DEFAULT_NUM_AREAS 1
>
> I'd drop this define, the magic 1 and a > 1 comparism seems to
> convey how it is used much better as the checks aren't about default
> or not, but about larger than one.
>
> I also think that we want some good way to size the default, e.g.
> by number of CPUs or memory size.

swiotlb_adjust_nareas() is exposed to platforms to set area number.
When swiotlb_init() is called, smp_init() isn't called at that point and
so standard API of checking cpu number (e.g, num_online_cpus()) doesn't
work. Platforms may have other ways to get cpu number(e.g x86 may ACPI
MADT table entries to get cpu nubmer) and set area number. I will post 
following patch to set cpu number via swiotlb_adjust_nareas(),

> > +void __init swiotlb_adjust_nareas(unsigned int nareas)
> > +{
> > +	if (!is_power_of_2(nareas)) {
> > +		pr_err("swiotlb: Invalid areas parameter %d.\n", nareas);
> > +		return;
> > +	}
> > +
> > +	default_nareas = nareas;
> > +
> > +	pr_info("area num %d.\n", nareas);
> > +	/* Round up number of slabs to the next power of 2.
> > +	 * The last area is going be smaller than the rest if
> > +	 * default_nslabs is not power of two.
> > +	 */
>
> Please follow the normal kernel comment style with a /* on its own line.

OK. Will update.