Re: [Xen-devel] [PATCH v3] xen/ns16550: Fix ISR lockup on Allwinner uart

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.10.17 at 13:44, <awais.masood@xxxxxxxxxx> wrote:
> This patch fixes an ISR lockup seen on Allwinner uart
> 
> On Allwinner H5, serial driver goes into an infinite loop
> when interrupts are enabled. The reason is a residual
> "busy detect" interrupt. Since the condition UART_IIR_NOINT
> will not be true unless this interrupt is cleared, the
> interrupt handler will remain locked up in this while loop.
> 
> A HW quirk fix was previously added for designware uart under
> commit:
> 50417cd978aa54930d065ac1f139f935d14af76d
> 
> It checks for a busy condition during setup and clears the
> condition by reading UART_USR register.
> 
> On Allwinner hardware, the "busy detect" condition occurs
> later because an LCR write is performed during setup 'after'
> this clear and if uart is busy, the "busy detect" condition
> will trigger again and cause the ISR lockup.
> 
> To solve this problem, the same UART_USR read operation needs
> to be performed within the interrupt handler to clear this
> condition.
> 
> Linux dw 8250 driver also handles this condition within
> interrupt handler
> http://elixir.free-electrons.com/linux/latest/source/drivers/tty/serial/8250/ 
> 8250_dw.c#L233
> 
> Tested on Orange Pi PC2 (H5). This issue is seen on H3
> as well and the same fix works.
> 
> Signed-off-by: Awais Masood <awais.masood@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>



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