linux-kernel - Re: [PATCH v2 01/11] mm/ioremap: change the return value of io[re|un]map

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Date:   Sun, 28 Aug 2022 17:55:52 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, hch@...radead.org,
        wangkefeng.wang@...wei.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 01/11] mm/ioremap: change the return value of
 io[re|un]map_allowed and rename

On 08/28/22 at 10:36am, Alexander Gordeev wrote:
> On Sat, Aug 20, 2022 at 08:31:15AM +0800, Baoquan He wrote:
> 
> Hi Baoquan,
> 
> >  arch_ioremap() return a bool,
> >    - IS_ERR means return an error
> >    - NULL means continue to remap
> >    - a non-NULL, non-IS_ERR pointer is returned directly
> >  arch_iounmap() return a bool,
> >    - 0 means continue to vunmap
> >    - error code means skip vunmap and return directly
> 
> It would make more sense if the return values were described
> from the prospective of an architecture, not the caller.
> I.e true - unmapped, false - not supported, etc.

Yes, sounds reasonable to me, thanks.

While ChristopheL suggested to take another way. Please see below link.
I will reply to Christophe to discuss that.

https://lore.kernel.org/all/8df89136-a7f2-9b66-d522-a4fb9860bf22@csgroup.eu/T/#u

If the current arch_ioremap() way is taken, I will change the
description as you said.

> 
> > diff --git a/mm/ioremap.c b/mm/ioremap.c
> > index 8652426282cc..99fde69becc7 100644
> > --- a/mm/ioremap.c
> > +++ b/mm/ioremap.c
> > @@ -17,6 +17,13 @@ void __iomem *ioremap_prot(phys_addr_t phys_addr, size_t size,
> >  	unsigned long offset, vaddr;
> >  	phys_addr_t last_addr;
> >  	struct vm_struct *area;
> > +	void __iomem *ioaddr;
> > +
> > +	ioaddr = arch_ioremap(phys_addr, size, prot);
> > +	if (IS_ERR(ioaddr))
> > +		return NULL;
> > +	else if (ioaddr)
> > +		return ioaddr;
> 
> It seems to me arch_ioremap() could simply return an address
> or an error. Then IOMEM_ERR_PTR(-ENOSYS) if the architecture
> does not support it reads much better than the cryptic NULL.

I may not follow. Returning NULL means arch_ioremap() doesn't give out a
mapped address and doesn't encounter wrong thing. NULL is a little
twisting, maybe '0' is better?

> 
> Probably arch_iounmap() returning error would look better too,
> though not sure about that.

Don't follow either. arch_iounmap() is returning error now.