Date: Mon, 22 Mar 2010 08:39:12 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org, gary.jennejohn@freenet.de Cc: Alexander Motin <mav@freebsd.org>, FreeBSD-Current <freebsd-current@freebsd.org> Subject: Re: Increasing MAXPHYS Message-ID: <201003220839.12907.jhb@freebsd.org> In-Reply-To: <20100322124018.7430f45e@ernst.jennejohn.org> References: <4BA4E7A9.3070502@FreeBSD.org> <4BA6517C.3050509@FreeBSD.org> <20100322124018.7430f45e@ernst.jennejohn.org>
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On Monday 22 March 2010 7:40:18 am Gary Jennejohn wrote: > On Sun, 21 Mar 2010 19:03:56 +0200 > Alexander Motin <mav@FreeBSD.org> wrote: > > > Scott Long wrote: > > > Are there non-CAM drivers that look at MAXPHYS, or that silently assume that > > > MAXPHYS will never be more than 128k? > > > > That is a question. > > > > I only did a quick&dirty grep looking for MAXPHYS in /sys. > > Some drivers redefine MAXPHYS to be 512KiB. Some use their own local > MAXPHYS which is usually 128KiB. > > Some look at MAXPHYS to figure out other things; the details escape me. > > There's one driver which actually uses 100*MAXPHYS for something, but I > didn't check the details. > > Lots of them were non-CAM drivers AFAICT. The problem is the drivers that _don't_ reference MAXPHYS. The driver author at the time "knew" that MAXPHYS was 128k, so he did the MAXPHYS-dependent calculation and just put the result in the driver (e.g. only supporting up to 32 segments (32 4k pages == 128k) in a bus dma tag as a magic number to bus_dma_tag_create() w/o documenting that the '32' was derived from 128k or what the actual hardware limit on nsegments is). These cannot be found by a simple grep, they require manually inspecting each driver. -- John Baldwin
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