Re: [PATCH v3 15/22] mtd: rawnand: fsmc: Stop implementing ->select_chip()

[Boris Brezillon <bbrezillon@xxxxxxxxxx>]

On Wed, 9 Jan 2019 23:20:40 +0100
Marc Gonzalez <marc.w.gonzalez@xxxxxxx> wrote:

> On 09/01/2019 20:44, Boris Brezillon wrote:
> 
> > Might be caused by a missing barrier: when de-asserting the CE line, we
> > must make sure all accesses to the ->data_va range have been done.
> > Can you try with the following diff applied?
> >   
> > --->8---  
> > diff --git a/drivers/mtd/nand/raw/fsmc_nand.c b/drivers/mtd/nand/raw/fsmc_nand.c
> > index 325b4414dccc..264d809c2d37 100644
> > --- a/drivers/mtd/nand/raw/fsmc_nand.c
> > +++ b/drivers/mtd/nand/raw/fsmc_nand.c
> > @@ -598,16 +598,21 @@ static void fsmc_ce_ctrl(struct fsmc_nand_data *host, bool assert)
> >  {
> >         u32 pc = readl(host->regs_va + FSMC_PC);
> >  
> > -       if (!assert)
> > +       if (!assert) {
> > +               /*
> > +                * Make sure all previous read/write have been done before
> > +                * de-asserting the CE line.
> > +                */
> > +               mb();
> >                 writel_relaxed(pc & ~FSMC_ENABLE, host->regs_va + FSMC_PC);
> > -       else
> > +       } else {
> >                 writel_relaxed(pc | FSMC_ENABLE, host->regs_va + FSMC_PC);
> > -
> > -       /*
> > -        * nCE line changes must be applied before returning from this
> > -        * function.
> > -        */
> > -       mb();
> > +               /*
> > +                * nCE assertion must be applied before returning from this
> > +                * function.
> > +                */
> > +               mb();
> > +       }  
> 
> + Will Deacon, an expert on arm mmio shenanigans.
> e.g. https://elinux.org/images/a/a8/Uh-oh-Its-IO-Ordering-Will-Deacon-Arm.pdf
> 
> As far as I understand (which isn't very far), a memory barrier (however
> strong) cannot guarantee that all (posted) writes have reached a device
> (unless the underlying bus explicitly provides such guarantees?)
> 
> At best, a memory barrier guarantees that the mmio writes have "left" the CPU
> and its "coherency fabric" (shared LLC). Thus subsequent accesses are
> guaranteed to appear "later" on the bus. (Much hand-waving sorry)

Yes, that's what I was trying to enforce.


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