Re: [PATCH v2 2/2] serial: 8250: Add new 8250-core based Broadcom STB driver

[Al Cooper]

I took another look at using the DMA framework for this and I still
don't think it fits. The problem is that the DMA hardware is so
tightly coupled to the 8250 UART hardware that a generic DMA model
where a DMA engine will read the data from the UART data register when
Data Ready is asserted and transfer it to memory, doesn't fit.  For
example:
- The flush timeout, parity error, framing error and overrun errors
are kept separately in the DMA hardware.
- The DMA hardware expects to be enabled before receiving input data
and it will continually write the data to ping pong data buffers as
long as a buffer empty bit is set on the next buffer.
- When DMA is enabled it bypasses some of the normal 8250 control and
data registers.

I think that trying to force this hardware into the current 8250 DMA
model using the DMA framework would not be a good approach.
I'll look at the other suggestions.

Thanks
Al



On Wed, Jan 20, 2021 at 11:47 AM Andy Shevchenko
<andy.shevchenko@xxxxxxxxx> wrote:
>
> On Tue, Jan 19, 2021 at 8:16 PM Florian Fainelli <f.fainelli@xxxxxxxxx> wrote:
> > On 1/19/2021 7:21 AM, Andy Shevchenko wrote:
> > > On Fri, Jan 15, 2021 at 11:19 PM Al Cooper <alcooperx@xxxxxxxxx> wrote:
> > >>
> > >> Add a UART driver for the new Broadcom 8250 based STB UART. The new
> > >> UART is backward compatible with the standard 8250, but has some
> > >> additional features. The new features include a high accuracy baud
> > >> rate clock system and DMA support.
> > >>
> > >> The driver will use the new optional BAUD MUX clock to select the best
> > >> one of the four master clocks (81MHz, 108MHz, 64MHz and 48MHz) to feed
> > >> the baud rate selection logic for any requested baud rate.  This allows
> > >> for more accurate BAUD rates when high speed baud rates are selected.
> > >>
> > >> The driver will use the new UART DMA hardware if the UART DMA registers
> > >> are specified in Device Tree "reg" property. The DMA functionality can
> > >> be disabled on kernel boot with the argument:
> > >> "8250_bcm7271.disable_dma=Y".
> > >>
> > >> The driver also set the UPSTAT_AUTOCTS flag when hardware flow control
> > >> is enabled. This flag is needed for UARTs that don't assert a CTS
> > >> changed interrupt when CTS changes and AFE (Hardware Flow Control) is
> > >> enabled.
> > >>
> > >> The driver also contains a workaround for a bug in the Synopsis 8250
> > >> core. The problem is that at high baud rates, the RX partial FIFO
> > >> timeout interrupt can occur but there is no RX data (DR not set in
> > >> the LSR register). In this case the driver will not read the Receive
> > >> Buffer Register, which clears the interrupt, and the system will get
> > >> continuous UART interrupts until the next RX character arrives. The
> > >> fix originally suggested by Synopsis was to read the Receive Buffer
> > >> Register and discard the character when the DR bit in the LSR was
> > >> not set, to clear the interrupt. The problem was that occasionally
> > >> a character would arrive just after the DR bit check and a valid
> > >> character would be discarded. The fix that was added will clear
> > >> receive interrupts to stop the interrupt, deassert RTS to insure
> > >> that no new data can arrive, wait for 1.5 character times for the
> > >> sender to react to RTS and then check for data and either do a dummy
> > >> read or a valid read. Sysfs error counters were also added and were
> > >> used to help create test software that would cause the error condition.
> > >> The counters can be found at:
> > >> /sys/devices/platform/rdb/*serial/rx_bad_timeout_late_char
> > >> /sys/devices/platform/rdb/*serial/rx_bad_timeout_no_char
> > >
> > > Brief looking into the code raises several questions:
> > >  - is it driver from the last decade?
> >
> > Work on this driver started back in 2018, that was indeed the last decade.
> >
> > >  - why it's not using what kernel provides?
> > >  - we have a lot of nice helpers:
> > >    - DMA Engine API
> >
> > Not sure this makes sense, given that the DMA hardware that was added to
> > this UART block is only used by the UART block and no other pieces of HW
> > in the system, nor will they ever be. Not sure it makes sense to pay the
> > cost of an extra indirection and subsystem unless there are at least two
> > consumers of that DMA hardware to warrant modeling it after a dmaengine
> > driver. I also remember that Al researched before whether 8250_dma.c
> > could work, and came to the conclusion that it would not, but I will let
> > him comment on the specifics.
>
> I see. In any case I still believe that the driver can be shrinked by
> a notable amount of lines.
>
> > >    - BIT() and GENMASK() macros
> > >    - tons of different helpers like regmap API (if you wish to dump
> > > registers via debugfs)
> > >
> > > Can you shrink this driver by 20-30% (I truly believe it's possible)
> > > and split DMA driver to drivers/dma (which may already have something
> > > similar there)?
> >
> > See previous response.
>
>
> --
> With Best Regards,
> Andy Shevchenko

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