Re: [RFC] With 8250 Designware UART, if writes to the LCR failed the kernel will hung up

From: Andy Shevchenko
Date: Fri Mar 13 2015 - 11:36:33 EST

On Tue, Mar 10, 2015 at 4:47 AM, Tim Kryger <tim.kryger@xxxxxxxxx> wrote:
> On Mon, Mar 9, 2015 at 8:05 AM, Alan Cox <alan@xxxxxxxxxxxxxxx> wrote:
>> Ah no - I meant what is their official software workaround for existing
>> parts with the bug ? Presumably they have an errata document that
>> discusses this and the correct methods they recommend to avoid the
>> hang ?
> As far as I know, the only advice they provided was rather naive.
> The documentation I saw suggested stashing a copy of the LCR and then
> rewriting it when the special LCR write failed interrupt was raised.
> That approach was not workable as the LCR might be written while the
> interrupt is masked causing the sequence of register writes to occur
> in an order other than what was desired.
> Additionally, when the LCR needed to be re-written but the UART stayed
> busy, the interrupt would keep firing and the driver would starve out
> everything else on the CPU.
> The current workaround of clearing fifos and retrying a fixed number
> of times isn't ideal but I'm not sure what else can be done given the
> way this hardware works.
> Additional background is in c49436b657d0a56a6ad90d14a7c3041add7cf64d

I hit the similar problem as Zhang described quite long ago when I
tried to write to the port chosen as console.
In my case seems the same (DW) IP is enumerated via PCI. Do we need to
tweak PCI cases as well?

With Best Regards,
Andy Shevchenko
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