linux-kernel - Re: [PATCH v7 04/12] powerpc/vas: Define helpers to access MMIO regions

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Date:   Sun, 27 Aug 2017 21:36:07 -0700
From:   Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        mikey@...ling.org, stewart@...ux.vnet.ibm.com, apopple@....ibm.com,
        hbabu@...ibm.com, oohall@...il.com, linuxppc-dev@...abs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 04/12] powerpc/vas: Define helpers to access MMIO
 regions

Michael Ellerman [mpe@...erman.id.au] wrote:
> Hi Suka,
> 
> Comments inline.
> 
> Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com> writes:
> > diff --git a/arch/powerpc/platforms/powernv/vas-window.c b/arch/powerpc/platforms/powernv/vas-window.c
> > index 6156fbe..a3a705a 100644
> > --- a/arch/powerpc/platforms/powernv/vas-window.c
> > +++ b/arch/powerpc/platforms/powernv/vas-window.c
> > @@ -9,9 +9,182 @@
> >  
> >  #include <linux/types.h>
> >  #include <linux/mutex.h>
> > +#include <linux/slab.h>
> > +#include <linux/io.h>
> >  
> >  #include "vas.h"
> >  
> > +/*
> > + * Compute the paste address region for the window @window using the
> > + * ->paste_base_addr and ->paste_win_id_shift we got from device tree.
> > + */
> > +void compute_paste_address(struct vas_window *window, uint64_t *addr, int *len)
> > +{
> > +	uint64_t base, shift;
> 
> Please use the kernel types, so u64 here.

Ok.

> 
> > +	int winid;
> > +
> > +	base = window->vinst->paste_base_addr;
> > +	shift = window->vinst->paste_win_id_shift;
> > +	winid = window->winid;
> > +
> > +	*addr  = base + (winid << shift);
> > +	if (len)
> > +		*len = PAGE_SIZE;
> 
> Having multiple output parameters makes for a pretty awkward API. Is it
> really necesssary given len is a constant PAGE_SIZE anyway.
> 
> If you didn't return len, then you could just make the function return
> the addr, and you wouldn't need any output parameters.

I agree, I went back and forth on it. I was trying to avoid callers
making assumptions on the size. But since there are just a couple
of places, I guess we could have them assume PAGE_SIZE.

> 
> One of the callers that passes len is unmap_paste_region(), but that
> is a bit odd. It would be more natural I think if once a window is
> mapped it knows its size. Or if the mapping will always just be one page
> then we can just know that.

Agree, since the len values are constant I was trying to avoid saving
them in each of the 64K windows - so the compute during unmap. Will change
to assume  PAGE_SIZE.

Also agree with other comments here.