stm32/adc: Optimise read_timed_multi() by caching buffer pointers.

dpgeorge · dpgeorge · commit aebd9701a78d · 2018-04-11T14:09:09.000+10:00
diff --git a/docs/library/pyb.ADC.rst b/docs/library/pyb.ADC.rst
@@ -125,9 +125,10 @@ Methods
 
        The maximum rate depends on factors including the data width and the
        number of ADC's being read. In testing two ADC's were sampled at a timer
-       rate of 140KHz without overrun. Samples were missed at 180KHz. At high
-       sample rates disabling interrupts for the duration can reduce the risk
-       of sporadic data loss.
+       rate of 210kHz without overrun. Samples were missed at 215kHz.  For three
+       ADC's the limit is around 140kHz, and for four it is around 110kHz.
+       At high sample rates disabling interrupts for the duration can reduce the
+       risk of sporadic data loss.
 
 The ADCAll Object
 -----------------
diff --git a/ports/stm32/adc.c b/ports/stm32/adc.c
@@ -475,12 +475,14 @@ STATIC mp_obj_t adc_read_timed_multi(mp_obj_t adc_array_in, mp_obj_t buf_array_i
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(buf_array[0], &bufinfo, MP_BUFFER_WRITE);
     size_t typesize = mp_binary_get_size('@', bufinfo.typecode, NULL);
+    void *bufptrs[nbufs];
     for (uint array_index = 0; array_index < nbufs; array_index++) {
         mp_buffer_info_t bufinfo_curr;
         mp_get_buffer_raise(buf_array[array_index], &bufinfo_curr, MP_BUFFER_WRITE);
         if ((bufinfo.len != bufinfo_curr.len) || (bufinfo.typecode != bufinfo_curr.typecode)) {
             mp_raise_ValueError("size and type of buffers must match");
         }
+        bufptrs[array_index] = bufinfo_curr.buf;
     }
 
     // Use the supplied timer object as the sampling time base
@@ -541,9 +543,7 @@ STATIC mp_obj_t adc_read_timed_multi(mp_obj_t adc_array_in, mp_obj_t buf_array_i
             if (typesize == 1) {
                 value >>= 4;
             }
-            mp_buffer_info_t bufinfo_curr;  // Get buf for current ADC
-            mp_get_buffer_raise(buf_array[array_index], &bufinfo_curr, MP_BUFFER_WRITE);
-            mp_binary_set_val_array_from_int(bufinfo_curr.typecode, bufinfo_curr.buf, elem_index, value);
+            mp_binary_set_val_array_from_int(bufinfo.typecode, bufptrs[array_index], elem_index, value);
         }
     }
 

Original file line number	Diff line number	Diff line change
`@@ -475,12 +475,14 @@ STATIC mp_obj_t adc_read_timed_multi(mp_obj_t adc_array_in, mp_obj_t buf_array_i`
`475`	`475`	`mp_buffer_info_t bufinfo;`
`476`	`476`	`mp_get_buffer_raise(buf_array[0], &bufinfo, MP_BUFFER_WRITE);`
`477`	`477`	`size_t typesize = mp_binary_get_size('@', bufinfo.typecode, NULL);`
	`478`	`+ void *bufptrs[nbufs];`
`478`	`479`	`for (uint array_index = 0; array_index < nbufs; array_index++) {`
`479`	`480`	`mp_buffer_info_t bufinfo_curr;`
`480`	`481`	`mp_get_buffer_raise(buf_array[array_index], &bufinfo_curr, MP_BUFFER_WRITE);`
`481`	`482`	`if ((bufinfo.len != bufinfo_curr.len) \|\| (bufinfo.typecode != bufinfo_curr.typecode)) {`
`482`	`483`	`mp_raise_ValueError("size and type of buffers must match");`
`483`	`484`	`}`
	`485`	`+ bufptrs[array_index] = bufinfo_curr.buf;`
`484`	`486`	`}`
`485`	`487`
`486`	`488`	`// Use the supplied timer object as the sampling time base`
`@@ -541,9 +543,7 @@ STATIC mp_obj_t adc_read_timed_multi(mp_obj_t adc_array_in, mp_obj_t buf_array_i`
`541`	`543`	`if (typesize == 1) {`
`542`	`544`	`value >>= 4;`
`543`	`545`	`}`
`544`		`- mp_buffer_info_t bufinfo_curr; // Get buf for current ADC`
`545`		`- mp_get_buffer_raise(buf_array[array_index], &bufinfo_curr, MP_BUFFER_WRITE);`
`546`		`- mp_binary_set_val_array_from_int(bufinfo_curr.typecode, bufinfo_curr.buf, elem_index, value);`
	`546`	`+ mp_binary_set_val_array_from_int(bufinfo.typecode, bufptrs[array_index], elem_index, value);`
`547`	`547`	`}`
`548`	`548`	`}`
`549`	`549`