Re: [RFC 3/9] MIPS: add virt_to_phys() and phys_to_virt()

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Sep 04, 2015 at 10:27:40AM +0300, Antony Pavlov wrote:
> On Fri, 4 Sep 2015 08:20:48 +0200
> Sascha Hauer <s.hauer@pengutronix.de> wrote:
> 
> > On Fri, Aug 28, 2015 at 06:46:14PM +0300, Antony Pavlov wrote:
> > > On Fri, 28 Aug 2015 08:34:32 +0200
> > > Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > > 
> > > > On Fri, Aug 28, 2015 at 01:24:04AM +0300, Antony Pavlov wrote:
> > > > > N.B. phys_to_virt() translates phys address
> > > > > to KSEG1 (uncached) address as barebox mips
> > > > > has no cache support.
> > > > 
> > > > What would it take to implement cache support for mips?
> > > lack of the cache support is critical problem for current barebox mips support.
> > > I'm planning to add cache support in several weeks.
> > > This task needs much test efforts for different boards.
> > > 
> > > Anyway I can't carry out cache adding work at one.
> > > But adding virt_to_phys and DMA support will help to add cache support one day anyway.
> > 
> > Looking at this again the virt_to_phys/phys_to_virt macros are not
> > necessary. dma_alloc_coherent() already returns both the virtual address
> > and the DMA address. It should be possible to replace DMA_ADDRESS_BROKEN
> > in the ehci driver with a real pointer and use it appropriatly in the
> > driver.
> 
> Here is a fragment of drivers/usb/host/ehci-hcd.c:
> 
>     while (td != (void *)QT_NEXT_TERMINATE) {
>            qh->qt_next = td->qt_next;
>            td = (void *)hc32_to_cpu(qh->qt_next);
>     }
> 
> 
> qh->qt_next contains physical memory address for EHCI controller,
> so after 'td = (void *)hc32_to_cpu(qh->qt_next)' contains physical address.
> 
> But latter we can use 'td->qt_next'. If physical address != virtual address
> then we will fail. So we need addition phys_to_virt() call here.

The problem with the ehci driver is that it uses qt_next pointers in the
driver to iterate over the TDs. qt_next is meant for the ehci hardware,
not for the software. What we need is an additional struct qTD *vqt_next
field in both struct qTD and struct QH for software iteration over the
list of TDs.

Sascha

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