linux-kernel - Re: [PATCH v1 3/9] parisc: Convert DMA map_page to map

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Message-ID: <20251003172618.GJ3360665@nvidia.com>
Date: Fri, 3 Oct 2025 14:26:18 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: John David Anglin <dave.anglin@...l.net>
Cc: Leon Romanovsky <leon@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Andreas Larsson <andreas@...sler.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"David S. Miller" <davem@...emloft.net>,
	Geoff Levand <geoff@...radead.org>, Helge Deller <deller@....de>,
	Ingo Molnar <mingo@...hat.com>, iommu@...ts.linux.dev,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	Jason Wang <jasowang@...hat.com>, Juergen Gross <jgross@...e.com>,
	linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Matt Turner <mattst88@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Richard Henderson <richard.henderson@...aro.org>,
	sparclinux@...r.kernel.org,
	Stefano Stabellini <sstabellini@...nel.org>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	virtualization@...ts.linux.dev, x86@...nel.org,
	xen-devel@...ts.xenproject.org, Magnus Lindholm <linmag7@...il.com>
Subject: Re: [PATCH v1 3/9] parisc: Convert DMA map_page to map_phys interface

On Fri, Oct 03, 2025 at 01:18:32PM -0400, John David Anglin wrote:
> On 2025-10-03 11:01 a.m., Jason Gunthorpe wrote:
> > This doesn't actually use the virt at all:
> > 
> > 	offset = ((unsigned long) addr) & ~IOVP_MASK;
> > 	if((size % L1_CACHE_BYTES) || ((unsigned long)addr % L1_CACHE_BYTES))
> > 		ccio_io_pdir_entry(pdir_start, KERNEL_SPACE, (unsigned long)addr, hint);
> > 
> > And ccio_io_pdir_entry():
> > 	pa = lpa(vba);
> > 
> > Is a special instruction that uses virt but AI tells me that special
> > LPA instruction is returning phys. Not sure if that is a different
> > value than virt_to_phys()..
> 
> ccio_io_pdir_entry currently only supports KERNEL_SPACE.  For KERNEL_SPACE, lpa() and
> virt_to_phys() are equivalent if page is mapped.  lpa() returns 0 if a non-access data
> TLB fault occurs (i.e., page isn't mapped).  Not sure if that
> matters.

After unfolding everything the expression is

  lpa(phys_to_virt(pa))

So if that is always equal to pa then lets just drop it.

phys_to_virt() always returns something kernel mapped, and it can't be
unmapped.

Jason