From 34cce4628b8509dc894735cdc5de8b10db21873a Mon Sep 17 00:00:00 2001 From: Ian Abbott Date: Fri, 5 Jun 2015 18:30:34 +0100 Subject: [PATCH] staging: comedi: das08: clarify sign-magnitude conversion For DAS08/JR/16 and DAS08/JR/AO-16 boards, the 16-bit analog input readings are assumed to be in a sign-magnitude format and need converting to the COMEDI unsigned sample format. The expressions to do the conversion look a little messy. Use a local variable `magnitude` to make it easier to follow. Also, there seems to be some discrepancy between the manual for these boards and the COMEDI code. The manual implies that 0 is full-scale negative and 65535 is full-scale positive. However, the COMEDI code has used the sign-magnitude conversion for these boards since these two boards were initially supported by a patch from an external contributor to the COMEDI project back in 2001. Assume the code is correct for now, but add a comment to mention the discrepancy. Signed-off-by: Ian Abbott Reviewed-by: H Hartley Sweeten Signed-off-by: Greg Kroah-Hartman --- drivers/staging/comedi/drivers/das08.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) diff --git a/drivers/staging/comedi/drivers/das08.c b/drivers/staging/comedi/drivers/das08.c index f74c4b009d8b..6c9bd10243b7 100644 --- a/drivers/staging/comedi/drivers/das08.c +++ b/drivers/staging/comedi/drivers/das08.c @@ -228,11 +228,26 @@ static int das08_ai_insn_read(struct comedi_device *dev, } else if (thisboard->ai_encoding == das08_pcm_encode12) { data[n] = (msb << 8) + lsb; } else if (thisboard->ai_encoding == das08_encode16) { - /* FPOS 16-bit boards are sign-magnitude */ + /* + * "JR" 16-bit boards are sign-magnitude. + * + * XXX The manual seems to imply that 0 is full-scale + * negative and 65535 is full-scale positive, but the + * original COMEDI patch to add support for the + * DAS08/JR/16 and DAS08/JR/16-AO boards have it + * encoded as sign-magnitude. Assume the original + * COMEDI code is correct for now. + */ + unsigned int magnitude = lsb | ((msb & 0x7f) << 8); + + /* + * MSB bit 7 is 0 for negative, 1 for positive voltage. + * COMEDI 16-bit bipolar data value for 0V is 0x8000. + */ if (msb & 0x80) - data[n] = (1 << 15) | lsb | ((msb & 0x7f) << 8); + data[n] = (1 << 15) + magnitude; else - data[n] = (1 << 15) - (lsb | (msb & 0x7f) << 8); + data[n] = (1 << 15) - magnitude; } else { dev_err(dev->class_dev, "bug! unknown ai encoding\n"); return -1; -- 2.39.5