RE: [PATCH 05/12] net: mana: Set the DMA device max page size

[Long Li]

> Subject: Re: [PATCH 05/12] net: mana: Set the DMA device max page size
> 
> On Tue, May 17, 2022 at 07:32:51PM +0000, Long Li wrote:
> > > Subject: Re: [PATCH 05/12] net: mana: Set the DMA device max page
> > > size
> > >
> > > On Tue, May 17, 2022 at 02:04:29AM -0700, longli@xxxxxxxxxxxxxxxxx
> wrote:
> > > > From: Long Li <longli@xxxxxxxxxxxxx>
> > > >
> > > > The system chooses default 64K page size if the device does not
> > > > specify the max page size the device can handle for DMA. This do
> > > > not work well when device is registering large chunk of memory in
> > > > that a large page size is more efficient.
> > > >
> > > > Set it to the maximum hardware supported page size.
> > >
> > > For RDMA devices this should be set to the largest segment size an
> > > ib_sge can take in when posting work. It should not be the page size
> > > of MR. 2M is a weird number for that, are you sure it is right?
> >
> > Yes, this is the maximum page size used in hardware page tables.
> 
> As I said, it should be the size of the sge in the WQE, not the "hardware page
> tables"

This driver uses the following code to figure out the largest page size for memory registration with hardware:

page_sz = ib_umem_find_best_pgsz(mr->umem, PAGE_SZ_BM, iova);

In this function, mr->umem is created with ib_dma_max_seg_size() as its max segment size when creating its sgtable.

The purpose of setting DMA page size to 2M is to make sure this function returns the largest possible MR size that the hardware can take. Otherwise, this function will return 64k: the default DMA size.

Long

> 
> Jason