Hello,
I kept exploring the sources of rtl8139 NIC and stuck with the weird spot in
'rtl8139_hw_start()' (same both in a 2.4.x and 2.6.x). I don't post here
very detailed code, as I assume everybody here has access to linux source
tree :)
...
/* init Tx buffer DMA addresses */
for (i = 0; i < NUM_TX_DESC; i++)
RTL_W32_F (TxAddr0 + (i * 4), tp->tx_bufs_dma + (tp->tx_buf[i] -
tp->tx_bufs));
RTL_W32_F is a macro exposing into 'writel(tp->tx_bufs_dma +
(tp->tx_buf[i] - tp->tx_bufs), ...)' call. By the time of this call 'tx_buf'
has been properly initialised and contains addresses of 4 TX descriptors.
As I understand 'tx_bufs_dma' holds the physical address of DMA memory,
while 'tx_bufs' - kernel virtual address.
What I can't figure out is the point of this calculations: tp->tx_bufs_dma
+ (tp->tx_buf[i] - tp->tx_bufs)
Why can't we simply have 'tp->tx_bufs_dma + (i * 4)' ?
Thanks in advance.
---
Best regards, Roman
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