2-0 |
RX_SIZE_THRESH_EN |
RX packet sized based free buffer queue enables. These bits control whether or not the flow will compare the packet size received from the back end application against the RX_SIZE_THRESHN fields to determine which FDQ to allocate the SOP buffer from. Each bit in this field corresponds to 1 of the 3 potential size thresholds that can be compared against. Bit 0 corresponds to RX_SIZE_THRESH0 and bit 2 corresponds to RX_SIZE_THRESH2.
The bits in this field are encoded as follows:
- 0 = Do not use the threshold.
- 1 = Use the thresholds to select between the 4 different potential SOP FDQs.
If thresholds are to be used, the thresholds must be used starting at 0 and progressing to 2. If a single threshold is required, threshold 0 must be used. If 2 thresholds are required, 0 and 1 must be used. It is illegal to enable a higher threshold without enabling the lower thresholds. The following values are valid:
- 0 - no thresholds enabled.
- 1 - threshold 0 enabled, implies RX_FDQ0_SZ0,1_QNUM,QMGR are used.
- 3 - thresholds 0, 1 enabled, implies RX_FDQ0_SZ0,1,2_QNUM,QMGR are used.
- 7 - thresholds 0, 1, 2 enabled, implies RX_FDQ0_SZ0,1,2,3_QNUM,QMGR are used.
If none of the thresholds are enabled, the DMA controller in the port will allocate the SOP buffer from the queue specified by the RX_FDQ0_SZ0_QMGR and RX_FDQ0_SZ0_QNUM fields. Support for packet size based FDQ selection is optional. If the port does not implement this feature, the bits of this field will be hardcoded to 0 and will not be writable by the host.
NOTE: This functionality is available only if supported by the particular peripheral. The usual limitation is that the peripheral doesn’t have packet size information available for the SOP transaction, which is when FDQ selection must be performed. Examples of this feature not being available are: AIF2, BCP, and SRIO Type 9 messages.
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