spi-transfer: Keep CS asserted during long transfers

Long transfers need to be split into multiple chunks because of
limitations by the spidev kernel driver. If this happens, we need to
make sure that the CS line remains high between the different chunks.

Add code to split the transfer into chunks and ask the driver to not
deassert CS after the transfer. Technically, this is only an
optimization as concurrent access to another device might still cause
deselection. However, this should mean that devices work without having
to change the spidev module parameter.

Use the feature in the hope that it will work. However, print a message
(not a warning), to help with debugging in case someone does run into
issues because of this.

libfprint/fpi-spi-transfer.c

@@ -22,6 +22,11 @@
 #include <linux/spi/spidev.h>
 #include <errno.h>
 
+/* spidev can only handle the specified block size, which defaults to 4096. */
+#define SPIDEV_BLOCK_SIZE_PARAM "/sys/module/spidev/parameters/bufsiz"
+#define SPIDEV_BLOCK_SIZE_FALLBACK 4096
+static gsize block_size = 0;
+
 /**
  * SECTION:fpi-spi-transfer
  * @title: SPI transfer helpers
@@ -114,6 +119,28 @@ fpi_spi_transfer_new (FpDevice * device, int spidev_fd)
 
   g_assert (FP_IS_DEVICE (device));
 
+  if (G_UNLIKELY (block_size == 0))
+    {
+      g_autoptr(GError) error = NULL;
+      g_autofree char *contents = NULL;
+
+      block_size = SPIDEV_BLOCK_SIZE_FALLBACK;
+
+      if (g_file_get_contents (SPIDEV_BLOCK_SIZE_PARAM, &contents, NULL, &error))
+        {
+          block_size = MIN (g_ascii_strtoull (contents, NULL, 0), G_MAXUINT16);
+          if (block_size == 0)
+            {
+              block_size = SPIDEV_BLOCK_SIZE_FALLBACK;
+              g_warning ("spidev blocksize could not be decoded, using %" G_GSIZE_FORMAT, block_size);
+            }
+        }
+      else
+        {
+          g_message ("Failed to read spidev block size, using %" G_GSIZE_FORMAT, block_size);
+        }
+    }
+
   self = g_slice_new0 (FpiSpiTransfer);
   self->ref_count = 1;
 
@@ -283,6 +310,78 @@ transfer_finish_cb (GObject *source_object, GAsyncResult *res, gpointer user_dat
   callback (transfer, transfer->device, transfer->user_data, error);
 }
 
+static int
+transfer_chunk (FpiSpiTransfer *transfer, gsize full_length, gsize *transferred)
+{
+  struct spi_ioc_transfer xfer[2] = { 0 };
+  gsize skip = *transferred;
+  gsize len = 0;
+  int transfers = 0;
+  int status;
+
+  if (transfer->buffer_wr)
+    {
+      if (skip < transfer->length_wr && len < block_size)
+        {
+          xfer[transfers].tx_buf = (guint64) transfer->buffer_wr + skip;
+          xfer[transfers].len = MIN (block_size, transfer->length_wr - skip);
+
+          len += xfer[transfers].len;
+          skip += xfer[transfers].len;
+
+          transfers += 1;
+        }
+
+      /* How much we need to skip in the next transfer. */
+      skip -= transfer->length_wr;
+    }
+
+  if (transfer->buffer_rd)
+    {
+      if (skip < transfer->length_rd && len < block_size)
+        {
+          xfer[transfers].rx_buf = (guint64) transfer->buffer_rd + skip;
+          xfer[transfers].len = MIN (block_size, transfer->length_rd - skip);
+
+          len += xfer[transfers].len;
+          /* skip += xfer[transfers].len; */
+
+          transfers += 1;
+        }
+
+      /* How much we need to skip in the next transfer. */
+      /* skip -= transfer->length_rd; */
+    }
+
+  g_assert (transfers > 0);
+
+  /* We have not transferred everything; ask driver to not deselect the chip.
+   * Unfortunately, this is inherently racy in case there are further devices
+   * on the same bus. In practice, it is hopefully unlikely to be an issue,
+   * but print a message once to help with debugging.
+   */
+  if (full_length < *transferred + len)
+    {
+      static gboolean warned = FALSE;
+
+      if (!warned)
+        {
+          g_message ("Split SPI transfer. In case of issues, try increasing the spidev buffer size.");
+          warned = TRUE;
+        }
+
+      xfer[transfers - 1].cs_change = TRUE;
+    }
+
+  /* This ioctl cannot be interrupted. */
+  status = ioctl (transfer->spidev_fd, SPI_IOC_MESSAGE (transfers), xfer);
+
+  if (status >= 0)
+    *transferred += len;
+
+  return status;
+}
+
 static void
 transfer_thread_func (GTask        *task,
                       gpointer      source_object,
@@ -290,9 +389,9 @@ transfer_thread_func (GTask        *task,
                       GCancellable *cancellable)
 {
   FpiSpiTransfer *transfer = (FpiSpiTransfer *) task_data;
-  struct spi_ioc_transfer xfer[2];
-  int transfers = 0;
-  int status;
+  gsize full_length;
+  gsize transferred = 0;
+  int status = 0;
 
   if (transfer->buffer_wr == NULL && transfer->buffer_rd == NULL)
     {
@@ -303,26 +402,14 @@ transfer_thread_func (GTask        *task,
       return;
     }
 
-  memset (xfer, 0, sizeof (xfer));
-
+  full_length = 0;
   if (transfer->buffer_wr)
-    {
-      xfer[transfers].tx_buf = (guint64) transfer->buffer_wr;
-      xfer[transfers].len = transfer->length_wr;
-
-      transfers += 1;
-    }
-
+    full_length += transfer->length_wr;
   if (transfer->buffer_rd)
-    {
-      xfer[transfers].rx_buf = (guint64) transfer->buffer_rd;
-      xfer[transfers].len = transfer->length_rd;
+    full_length += transfer->length_rd;
 
-      transfers += 1;
-    }
-
-  /* This ioctl cannot be interrupted. */
-  status = ioctl (transfer->spidev_fd, SPI_IOC_MESSAGE (transfers), xfer);
+  while (transferred < full_length && status >= 0)
+    status = transfer_chunk (transfer, full_length, &transferred);
 
   if (status < 0)
     {