From: Linus Torvalds
> Sent: 07 January 2019 17:44
> On Mon, Jan 7, 2019 at 1:55 AM David Laight <david.lai...@aculab.com> wrote:
> >
> > I needed to open-code one part because it wants to do copy_to_user()
> > from a PCIe address buffer (which has to work).
>
> It will never work for memcpy_fromio(). Any driver that thinks it will
> copy from io space to user space absolutely *has* to do it by hand. No
> questions, and no exceptions. Some loop like
>
> for (..)
> put_user(readl(iomem++), uaddr++);
>
> because neither copy_to_user() nor memcpy_fromio() will *ever* handle
> that correctly.
>
> They might randomly happen to work on x86, but absolutely nowhere else.
Actually they tend to handle it on a lot of systems.
(But I don't do it.)
Probably most of those where vm_iomap_memory() (to map IO memory
space directly into user pages) works.
It might be 'interesting' to build an amd64 kernel where all the IO
memory addresses (eg returned by pci_iomap()) are offset by a
large constant so direct accesses all fault and all the readl()
macros (etc) add it back in.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT,
UK
Registration No: 1397386 (Wales)
