a-lunev commented on a change in pull request #4236:
URL: https://github.com/apache/incubator-nuttx/pull/4236#discussion_r677504222
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File path: drivers/wireless/ieee80211/bcm43xxx/bcmf_sdio.h
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@@ -110,13 +111,22 @@ struct bcmf_sdio_dev_s
/* Structure used to manage SDIO frames */
+struct __attribute__((packed)) bcmf_dma_buf
+{
+ uint8_t align_padding[CONFIG_IEEE80211_BROADCOM_SDIO_DMA_BUF_ALIGNMENT -
Review comment:
I'm referencing bcmf_dma_buf struct I've introduced. first_word[] and
remaining_aligned_buf[] parts should not have a gap between each other as they
form the whole bcmf frame. Thus I've added "attribute((packed))" to the struct
to prevent a gap inside of the struct (now I've wrapped it into
begin_packed_struct / end_packed_struct for compiler independence).
However, the whole bcmf_dma_buf struct should be aligned to the address
boundary of 16.
As the struct is packed inside and aligned to 16 outside, then
remaining_aligned_buf[] is also aligned to the address boundary of 16, that is
the purpose as it's used in DMA transfer.
There are two conditions at the same time:
- remaining_aligned_buf[] should be aligned;
- remaining_aligned_buf[] should follow first_word[] w/o a gap.
The main purpose of first_word[] is to receive the length (the first phase)
of the remaining frame to be read (the second phase). DMA receive transfer can
not be invoked w/o knowing the length of the frame first, thus it can not be a
single transfer.
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