[PATCH] staging: comedi: cb_pcidas: reinstate delay removed from trimpot setting

Ian Abbott abbotti at mev.co.uk
Mon Nov 2 11:16:05 UTC 2020


On 02/11/2020 10:25, Ian Abbott wrote:
> On 29/10/2020 14:18, Ian Abbott wrote:
>> Commit eddd2a4c675c ("staging: comedi: cb_pcidas: refactor
>> write_calibration_bitstream()") inadvertently removed one of the
>> `udelay(1)` calls when writing to the calibration register in
>> `cb_pcidas_calib_write()`.  Reinstate the delay.  It may seem strange
>> that the delay is placed before the register write, but this function is
>> called in a loop so the extra delay can make a difference.
>>
>> This _might_ solve reported issues reading analog inputs on a
>> PCIe-DAS1602/16 card where the analog input values "were scaled in a
>> strange way that didn't make sense".  On the same hardware running a
>> system with a 3.13 kernel, and then a system with a 4.4 kernel, but with
>> the same application software, the system with the 3.13 kernel was fine,
>> but the one with the 4.4 kernel exhibited the problem.  Of the 90
>> changes to the driver between those kernel versions, this change looked
>> like the most likely culprit.
> 
> Actually, I've realized that this patch will have no effect on the 
> PCIe-DAS1602/16 card because it uses a different driver - cb_pcimdas, 
> not cb_pcidas.

But that's also confusing because PCIe-DAS1602/16 was not supported 
until the 3.19 kernel!  I know the reported has both PCI-DAS1602/16 and 
PCIe-DAS1602/16 cards (supported by cb_pcidas and cb_pcimdas 
respectively), so there could have been some mix-up in the reporting.

> 
> Greg, you might as well drop this patch if you haven't already applied 
> it, since it was only a hunch that it fixed a problem.
> 


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