Re: [PATCH v2 1/2] sc16is7xx: Prevent TX buffer overrun, prevent crash

[Florian Achleitner <achleitner.florian@xxxxxxxxxxx>]

Hi,

some news.
On Monday 05 October 2015 14:34:01 Florian Achleitner wrote:
> I digged a little deeper and did some measurements to support my idea.
> I think the reason for the 255 read is that the chip does not support the
> zero length write.
> 
> The chip's SPI interface defines two sorts of frames. One for normal
> register access, which is essentially an address followed by one byte of
> data, either read or written.
> 
> The second type is for accessing the fifo. It has an address and two bytes
> of data, by definition.
> 
> If the master now issues a zero length write, it sends only the address
> byte, but the chip will expect two following data bytes, which do not
> arrive. Instead it will consume the following frame. When this frame is the
> tx fifo level read, the chip will not drive its SO line (still expecting a
> fifo write), and the master reads 255. Now two bytes were clocked, and they
> are back in sync. However, the value is crap.
> 
> If my theorie is true, we would also have to make sure, that fifo access is
> always two bytes to keep it synced. I will check this, and craft another
> patch, if neccessary.

My theorie is wrong with fifo frame lengths. Actually, they can be be of 
arbitrary length and the SPI chip select (CS) terminates a frame. Thus, the 
complete fifo can be filled at once, and single bytes can be written, as well.

However, a zero-length write seems to confuse the chip. If I prevent it, 
everything works.

As already mentioned, all regmap operations return success.

Florian

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