Re: [PATCH 3/4] ch341: Limit prescaler on HL340 variant

[Michael Hanselmann <public@hansmi.ch>]

On 24.03.20 11:41, Johan Hovold wrote:
> On Fri, Mar 06, 2020 at 07:00:44PM +0000, Michael Hanselmann wrote:
>> HL340 devices, a subset of all CH340 devices, do not work correctly when
>> the highest prescaler bit (0b100) is set. Limit these to the lower
>> prescaler values at the cost of timing precision.
> 
> When we discussed this off list, you said that your device could handle
> the highest prescaler bit being set for some rates. You specifically
> confirmed that the 576000 and 921600 rates worked, while 110, 134 and
> 200 did not.
> 
> Could you reconfirm which, if any, of the following rates work with the
> current driver?
> 
> 	    1152000
>              921600
>              576000
>                 200
>                 134
>                 110
> 
> Perhaps we can still continue supporting the higher rates, which are way
> off unless using the factor-2 prescaler.

You're right, 110, 134 and 200 don't work whereas 576000, 921600 and
1152000 do. I totally missed this while working on the patch.

I ended up doing more research and figured out that fact=0 with ps=0..3
works, as does fact=1 with ps=3. It's only fact=1 with ps0..2 which is
not working properly.

The next revision of the series will contain additional patches to
restructure the prescaler computation before implementing the quirk.

Michael