From c6657b831da9b4ee01bfd85f6dd356717c5f0c35 Mon Sep 17 00:00:00 2001
From: Johannes Demel <johannes.demel@ettus.com>
Date: Wed, 13 Nov 2013 09:31:54 -0800
Subject: sc12: fixed byte alignment issue
---
host/lib/convert/convert_unpack_sc12.cpp | 61 +++++++++++++++++++++++++++++---
1 file changed, 57 insertions(+), 4 deletions(-)
(limited to 'host')
diff --git a/host/lib/convert/convert_unpack_sc12.cpp b/host/lib/convert/convert_unpack_sc12.cpp
index f578b6c95..e98ab73f1 100644
--- a/host/lib/convert/convert_unpack_sc12.cpp
+++ b/host/lib/convert/convert_unpack_sc12.cpp
@@ -32,6 +32,17 @@ struct item32_sc12_3x
item32_t line2;
};
+/*
+ * convert_sc12_item32_3_to_star_4 takes in 3 lines with 32 bit each
+ * and converts them 4 samples of type 'std::complex<type>'.
+ * The structure of the 3 lines is as follows:
+ * _ _ _ _ _ _ _ _
+ * |_ _ _1_ _ _|_ _|
+ * |_2_ _ _|_ _ _3_|
+ * |_ _|_ _ _4_ _ _|
+ *
+ * The numbers mark the position of one complex sample.
+ */
template <typename type, tohost32_type tohost>
void convert_sc12_item32_3_to_star_4
(
@@ -84,17 +95,48 @@ struct convert_sc12_item32_1_to_star_1 : public converter
_scalar = scalar/unpack_growth;
}
+ /*
+ * This converter takes in 24 bits complex samples, 12 bits I and 12 bits Q, and converts them to type 'std::complex<type>'.
+ * 'type' is usually 'float'.
+ * For the converter to work correctly the used managed_buffer which holds all samples of one packet has to be 32 bits aligned.
+ * We assume 32 bits to be one line. This said the converter must be aware where it is supposed to start within 3 lines.
+ *
+ */
void operator()(const input_type &inputs, const output_type &outputs, const size_t nsamps)
{
- const item32_sc12_3x *input = reinterpret_cast<const item32_sc12_3x *>(size_t(inputs[0]) & ~0x3);
+ /*
+ * Looking at the line structure above we can identify 4 cases.
+ * Each corresponds to the start of a different sample within a 3 line block.
+ * head_samps derives the number of samples left within one block.
+ * Then the number of bytes the converter has to rewind are calculated.
+ */
+ const size_t head_samps = size_t(inputs[0]) & 0x3;
+ size_t rewind = 0;
+ switch(head_samps)
+ {
+ case 0: break;
+ case 1: rewind = 9; break;
+ case 2: rewind = 6; break;
+ case 3: rewind = 3; break;
+ }
+
+ /*
+ * The pointer *input now points to the head of a 3 line block.
+ */
+ const item32_sc12_3x *input = reinterpret_cast<const item32_sc12_3x *>(size_t(inputs[0]) - rewind);
std::complex<type> *output = reinterpret_cast<std::complex<type> *>(outputs[0]);
//helper variables
std::complex<type> dummy0, dummy1, dummy2;
size_t i = 0, o = 0;
- //handle the head case
- const size_t head_samps = size_t(inputs[0]) & 0x3;
+ /*
+ * handle the head case
+ * head_samps holds the number of samples left in a block.
+ * The 3 line converter is called for the whole block and already processed samples are dumped.
+ * We don't run into the risk of a SIGSEGV because input will always point to valid memory within a managed_buffer.
+ * Furthermore the bytes in a buffer remain unchanged after they have been copied into it.
+ */
switch (head_samps)
{
case 0: break; //no head
@@ -111,7 +153,18 @@ struct convert_sc12_item32_1_to_star_1 : public converter
i++; o += 4;
}
- //handle the tail case
+ /*
+ * handle the tail case
+ * The converter can be called with any number of samples to be converted.
+ * This can end up in only a part of a block to be converted in one call.
+ * We never have to worry about SIGSEGVs here as long as we end in the middle of a managed_buffer.
+ * If we are at the end of managed_buffer there are 2 precautions to prevent SIGSEGVs.
+ * Firstly only a read operation is performed.
+ * Secondly managed_buffers allocate a fixed size memory which is always larger than the actually used size.
+ * e.g. The current sample maximum is 2000 samples in a packet over USB.
+ * With sc12 samples a packet consists of 6000kb but managed_buffers allocate 16kb each.
+ * Thus we don't run into problems here either.
+ */
const size_t tail_samps = nsamps - o;
switch (tail_samps)
{
--
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