linux-kernel - Re: [PATCH 03/17] fpga: dfl: fme: support 512bit data width PR

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Date:   Mon, 25 Mar 2019 17:58:36 -0500
From:   Scott Wood <swood@...hat.com>
To:     Wu Hao <hao.wu@...el.com>, atull@...nel.org, mdf@...nel.org,
        linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     linux-api@...r.kernel.org, Ananda Ravuri <ananda.ravuri@...el.com>,
        Xu Yilun <yilun.xu@...el.com>
Subject: Re: [PATCH 03/17] fpga: dfl: fme: support 512bit data width PR

On Mon, 2019-03-25 at 17:53 -0500, Scott Wood wrote:
> On Mon, 2019-03-25 at 11:07 +0800, Wu Hao wrote:
> > In early partial reconfiguration private feature, it only
> > supports 32bit data width when writing data to hardware for
> > PR. 512bit data width PR support is an important optimization
> > for some specific solutions (e.g. XEON with FPGA integrated),
> > it allows driver to use AVX512 instruction to improve the
> > performance of partial reconfiguration. e.g. programming one
> > 100MB bitstream image via this 512bit data width PR hardware
> > only takes ~300ms, but 32bit revision requires ~3s per test
> > result.
> > 
> > Please note now this optimization is only done on revision 2
> > of this PR private feature which is only used in integrated
> > solution that AVX512 is always supported.
> > 
> > Signed-off-by: Ananda Ravuri <ananda.ravuri@...el.com>
> > Signed-off-by: Xu Yilun <yilun.xu@...el.com>
> > Signed-off-by: Wu Hao <hao.wu@...el.com>
> > ---
> >  drivers/fpga/dfl-fme-main.c |  3 ++
> >  drivers/fpga/dfl-fme-mgr.c  | 75 +++++++++++++++++++++++++++++++++++++-
> > --
> > -----
> >  drivers/fpga/dfl-fme-pr.c   | 45 ++++++++++++++++-----------
> >  drivers/fpga/dfl-fme.h      |  2 ++
> >  drivers/fpga/dfl.h          |  5 +++
> >  5 files changed, 99 insertions(+), 31 deletions(-)
> > 
> > diff --git a/drivers/fpga/dfl-fme-main.c b/drivers/fpga/dfl-fme-main.c
> > index 086ad24..076d74f 100644
> > --- a/drivers/fpga/dfl-fme-main.c
> > +++ b/drivers/fpga/dfl-fme-main.c
> > @@ -21,6 +21,8 @@
> >  #include "dfl.h"
> >  #include "dfl-fme.h"
> >  
> > +#define DRV_VERSION	"0.8"
> 
> What is this going to be used for?  Under what circumstances will the
> driver version be bumped?  What does it have to do with 512-bit writes?
> 
> > +#if defined(CONFIG_X86) && defined(CONFIG_AS_AVX512)
> > +
> > +#include <asm/fpu/api.h>
> > +
> > +static inline void copy512(void *src, void __iomem *dst)
> > +{
> > +	kernel_fpu_begin();
> > +
> > +	asm volatile("vmovdqu64 (%0), %%zmm0;"
> > +		     "vmovntdq %%zmm0, (%1);"
> > +		     :
> > +		     : "r"(src), "r"(dst));
> > +
> > +	kernel_fpu_end();
> > +}
> 
> Shouldn't there be some sort of check that AVX512 is actually supported
> on the running system?
> 
> Also, src should be const, and the asm statement should have a memory
> clobber.
> 
> > +#else
> > +static inline void copy512(void *src, void __iomem *dst)
> > +{
> > +	WARN_ON_ONCE(1);
> > +}
> > +#endif
> 
> Likewise, this will be called if a revision 2 device is used on non-x86
> (or on x86 with an old binutils).  The driver should fall back to 32-bit
> in such cases.

Sorry, I missed the comment about revision 2 only being on integrated
devices -- but will that always be the case?  Seems worthwhile to check for
AVX512 support anyway.  And there's still the possibility of being built
with an old binutils such that CONFIG_AS_AVX512 is not set, or running on a
kernel where avx512 was disabled via a boot option.

What about future revisions >= 2?  Currently the driver will treat them as
if they were revision < 2.  Is that intended?

-Scott