Re: [PATCH v2] drivers/firmware/broadcom/bcm47xx_nvram.c: fix incorrect __ioread32_copy

[Stephen Boyd <sboyd@codeaurora.org>]

On 03/23, Hauke Mehrtens wrote:
> On 03/16/2016 12:06 AM, Aaro Koskinen wrote:
> > Commit 1f330c327900 ("drivers/firmware/broadcom/bcm47xx_nvram.c: use
> > __ioread32_copy() instead of open-coding") switched to use a generic copy
> > function, but failed to notice that the header pointer is updated between
> > the two copies, resulting in bogus data being copied in the latter one.
> > Fix by keeping the old header pointer.
> > 
> > The patch fixes totally broken networking on WRT54GL router (both LAN
> > and WLAN interfaces fail to probe).
> > 
> > Fixes: 1f330c327900 ("drivers/firmware/broadcom/bcm47xx_nvram.c: use __ioread32_copy() instead of open-coding")
> > Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
> > ---
> > 
> > 	v2: Avoid using the device memory after the first copy when
> > 	    checking the nvram length, suggested by Stephen Boyd.
> > 
> > 	v1: http://marc.info/?t=145807850800003&r=1&w=2
> > 
> >  drivers/firmware/broadcom/bcm47xx_nvram.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/firmware/broadcom/bcm47xx_nvram.c b/drivers/firmware/broadcom/bcm47xx_nvram.c
> > index 0c2f0a6..0b631e5 100644
> > --- a/drivers/firmware/broadcom/bcm47xx_nvram.c
> > +++ b/drivers/firmware/broadcom/bcm47xx_nvram.c
> > @@ -94,15 +94,14 @@ static int nvram_find_and_copy(void __iomem *iobase, u32 lim)
> >  
> >  found:
> >  	__ioread32_copy(nvram_buf, header, sizeof(*header) / 4);
> > -	header = (struct nvram_header *)nvram_buf;
> > -	nvram_len = header->len;
> > +	nvram_len = ((struct nvram_header *)(nvram_buf))->len;
> 
> I do not understand why this change is needed? Doesn't the old code do
> exactly the same as the new one?
> 
> The old code updated the header pointer and then accesses a member, the
> new one directly accesses this member without updating this pointer.
> 
> I assume, I am missing something. ;-)

The goal is to access 'nvram_buf' which is a copy of 'header'.
This is to avoid any problems with accessing device memory, i.e.
'header', without using the appropriate I/O accessors (readl,
readw, readb).

The bug that's being fixed though is to make sure 'header'
doesn't get overwritten with the pointer to the in-memory copy
that we just made. Further down in this function we copy the
second 'header' that lives in device memory, and repointing
'header' to the in-memory copy breaks that.

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